Rat-kittens are born naked and rather unformed-looking. Their markings if any are visible on the skin after only a few days. They are fluffy by the time they are a week old, open their eyes at about 10 days and leave the nest a few days later.
It is easiest to sex them at around 2-3 weeks, when the fur is short enough to see the nipple-spots on the does, or after 5 weeks when the bucks' testicles drop. They are weaned at 4-5 weeks, and begin to moult into their smooth (unless they are rex), shiny adult coats at about 6 weeks, starting with the fur on the forehead and along the spine. Does are fertile at 5 weeks and bucks at 7 weeks.
For more details see section on reproduction.
They are fully adult at 6 months, but may continue to grow slowly throughout life. Dominant bucks produce a pheromone which actually supresses growth in their subordinates, so if the boss buck in a pack dies or is removed, or falls in the pecking order, the other males in the group will put on a noticeable growth-spurt.
Lifespan varies from strain to strain, but is generally between 20 months and 3½ years. 2¼ years is the norm, 7 years 4 months the record. Very approximately, for most rats one month of life is roughly equivalent to 3 years for a human.