As ageing sets in many rats change weight, becoming either much fatter or much thinner than they were in their prime. Thinness is often a sign of kidney and/or respiratory failure. Older rats are also more prone to tumours.
Old rats often become rubbery and uncoordinated, or even partially paralysed, due to a stroke or other neurological problem. In severe cases they will need a surface they can really get a grip on (which the usual plastic-and-shavings is not) in order to get about freely. Sections of old short-piled carpet probably provide the best traction, but quickly become fouled. VetBed - a type of absorbent fur-fabric obtainable from some pet-shops - is excellent as it absorbs urine, leaving the surface warm and dry, and is machine-washable. The only problem with VetBed is that the shaggy, slightly frizzy pile can snag the animal's claws if they are even slightly hooked, so you may need to trim them.
If the rat is lying down a lot and unable to groom itself properly it may develop sore skin due to urinating over its own thighs. Regular bathing is helpful, as is a non-greasy nappy-rash cream.
Whether or not they are living on VetBed, some elderly rats develop overgrown claws which will need to be trimmed every few weeks if they are not to snag and break, or even curl over and penetrate the pad.
Some elderly rats develop misaligned and/or overgrown teeth which have to be trimmed: see section on dental problems.
Even if their teeth look normal some older rats lose the ability to chew, due to stroke or other neurological problems, and have to be put on a soft diet. Tinned cat food, proprietary baby-foods (reconstituted powders such as Millupa are probably cheapest) and Readybrek (finely-powdered cooked oatmeal) are all good and not too expensive, but almost anything squashy - scrambled egg, chicken livers, slightly overcooked pasta, rice, porridge, mashed potato, raspberry jelly, neat marmalade - will be gratefully received. One of my old does lived for months on an exclusive diet of Ambrosia Devon Custard, and did very well on it.
Many old rats, especially bucks, suffer from kidney-failure and/or heart-failure, usually accompanied by weight-loss and wheezing. These heart/kidney/respiratory problems are so common and so inter-related that they have a separate page all to themselves, q.v..
Seriously old rats, especially if chesty, often have difficulties with their temperature control. Avoid letting the animal sleep too near a fire, as this can cause over-heating. The best thing for keeping a geriatric rat at the right temperature is probably a room-temperature of about 70-75°F and a heap of other rats.
At the same time, very old or sick rats may also become excessively cold, even in quite a warm room. In that case it is advisable to make up a bed near the fire, if you have one, while preventing the animal from getting so close it overheats. In severe cases you should lay the rat on a hot-water-bottle, or better still on an electric blanket or a heating-pad such as is sometimes used for puppies - but make sure the heat-source isn't uncomfortably hot, and that the rat can get away from it if it feels too warm. E.g. if the rat is in a small hospital cage, place a heating-pad under only half of the cage, so the rat can choose whether to lie directly over it or not.
Elderly rats, especially heavy bucks, sometimes develop a moist, spongy inflammation of the heels - see under pododermatitis.
Elderly or even middle-aged rats occasionally develop cateracts, visible as a cloudiness inside the eye - see section on eyesight.
Your geriatric rat, especially if he or she has lost strength or control from the hind legs, may become unable to clean inside his or her own ears. If these become infected your vet will be able to give you antibiotic drops, but as a precaution you should clean the ears out yourself a couple of times a week. Personally I use the point of my fingernail, which enables me to feel exactly what I'm doing, but a soft twist of tissue or cotton-wool would probably be OK. Don't use cotton-buds, as these can damage the ear-drum. You can also rinse the inside of the ear out with a squirt of Oterna or Dermisol, available from your vet, but as these sting I try not to use them unless I have to.
An old rat who has become unable to wash itself properly, especially one who has no cagemates to groom it and/or is on a soft diet and likes to wade in it up to the armpits, will need to be bathed every so often. Most rats don't object to this as much as you might expect, once they realize you aren't trying to drown them.
Male rats produce a thick, lanolin-like oil in the fur on their backs which they use for scent-marking: in some old bucks this ceases and the coat becomes dry and fluffy, but in others the back-fur can become clogged with thick orange grease - see section on coat-care for advice on this, and on bathing in general.
A certain amount of hair-loss is common in older rats, especially rexes who can end up almost completely bald. Many old rats end up looking distinctly moth-eaten, and in some you can clearly see the wrinkles which are underneath the fur of many rats even when young (some rats are just baggy under the fur: some look like a pink concertina). Sometimes the hair becomes so sparse that you can also see the roots of it growing in rows - rather as it does on the scalp of a balding doll.
If you turn a rat over and look at its belly, about ½" forwards of the genital papilla and ¼" out from the mid-line (depending on the size of the animal) on either side there is a small patch of skin which evidently has a different embryological origin to the skin around it - shown by the fact that in marked rats these patches can frequently be seen as a pair of coloured dots. The skin in these areas seem to be unusually thin and is a common site for small tears or ulcers in the elderly. Rinsing with tea-tree oil or Dermisol (an antiseptic wash obtainable from your vet - very effective, but stingy) once a day for a couple of days is usually enough to clear this up: if not, dress as for abscesses or see your vet for a topical (i.e. directly applied to the site of the problem) antibiotic. These thin patches of skin lie over the ureters and may be a built-in safety-valve of sorts: a prominent member of the British rat-fancy told me that a buck of hers who developed a blockage in his urethra opened up a stoma on this site and urinated through it quite happily, if incontinently.
Old bucks occasionally develop hard white granular lumps in the penile sheath, not to be confused with a slight lumpiness in the skin of the sheath itself - these are discrete gravel-like objects, made of solidified semen. They must be pretty uncomfortable and can cause difficulties in urination, so they need to be expressed by gentle squeezing. If a buck has this problem it is likely to recur, so he should be checked for it every day: often you can see whether there is a lump in the sheath or not just from the look of it.
Apart from respiratory problems rats are not often sick, but when they are they often die - not because they are weak in the face of infection (like guinea pigs) but because they are so resistant to infection that the mere fact that they have been able to become ill often means that their immune system is going down, and this will generally be followed by total systemic collapse and death. The following problems are all either frequently or generally symptomatic of impending death, and their presence or absence may have a bearing on whether or not you decide to e.g. have an animal operated on for a tumour, or indeed put down.
Creeping paralysis of the hind legs is the least threatening of these symptoms, as it can be due to stroke or arthritis or to a slow-acting neurological problem called degenerative myelopathy, and in these cases it has no great bearing on life span. There is a greatly increased likelihood that a rat whose hind legs have gone will be dead within a couple of weeks - but you might be lucky and he might be here this time next year.
Distended liver, whether due to a tumour or to an infection, is always fatal in my experience although the severity of the condition, and therefore the nearness of death, varies greatly. A very mild case might live six months in reasonable health, but a severe one is likely to be dead within the week. I have never found a vet who knows any treatment for liver disorders in rats - if you have, do let me know.
Heart failure, indicated by a blueness (other than any natural dark pigmentation) of the lips and the fingers and toes and by rapid breathing, is a variable sign. Give decongestants if the animal is at all chesty, and make sure it is getting enough fresh air: expect death in anything between two months and two minutes, according to the degree of blueness etc.. Treatment with ACE inhibitors, as described in the heart/kidney/respiratory section, may prolong the animal's life - or at least make it more comfortable.
The quickest way to tell whether rapid breathing is due to straightforward heart-failure or to a respiratory problem, incidentally, is to stuff the rat's nose down your own ear and listen for any wheezing, roaring or snuffling noises.
Clouding of the surface of the eye is another bad sign - not to be confused with cataracts, which are seen as white or smoky patches deep inside the eye and which, while commonest in old rats, have no bearing on life-span. If the surface of the eye takes on a cloudy, matte appearance ask your vet for drops in case this is due to an eye infection - if it isn't, the animal will probably be dead in three or four weeks. However I have seen one case where a doe whose health was being pulled down by a large tumour developed cloudy eyes which cleared again after her operation, so it isn't an invariable terminal sign - only nearly so.
Abscesses around the head often indicate immune failure or a brain tumour - for methods of treatment see under infections. Abscesses around the roots of the teeth are quite a bad sign. Abscesses behind the eyes, which often form at the same time as those on the teeth, are a very bad sign: expect a high probability of death within a fortnight, even if you manage to clear up the infection.
Necrosis - death and decay - of the tail-tip, in which the end of the tail turns black and cold (not to be confused with a bad bruise, which would be black and hot), is an extremely bad sign. Consult your vet as to whether or not the affected part needs to be amputated, but this symptom is indicative of circulatory failure and I would normally expect death within a week.