Rats are very undeveloped when they are born, being completely naked, blind and dependent on their mother, who needs to be fed a high nutrient diet while nursing them. At around two weeks their eyes open, and they start to leave the nest and explore the cage soon after this. They will begin to try solid food by three weeks old, and enjoy high protein/carb weaning mixes, but are still taking a lot of nutrition from their mother. Males should be removed from their mother and sisters before five weeks old, when the babies have been known to become fertile. The females can stay with mum for now. Both sexes benefit very much from another couple of weeks of familiar care before homing, so are usually homed at six or seven weeks, or even later if they are small for their age.
Rats are classed as 'kittens' up until 13 weeks of age, although they continue growing until 6 to 8 months old and bucks sometimes muscle up more after this. Bucks often stop taking as much exercise after this age, and with both sexes you need to be careful not to let them become overweight, as this will shorten their lives.
There is a large variation from rat to rat as to when they begin to look old. The average age for rats in the UK is around two years, but it's not uncommon to lose an 18 month old and, of course, many rats reach 2.5 years or more. The belief that dumbo eared rats live longer than top eared rats is a myth, the only difference being the ear position. Older rats become more prone to respiratory problems, mammary tumours and hind leg degeneration, and can benefit from a diet with slightly lower levels of protein to reduce the load on their kidneys.