It's best to choose a rat 4-6 weeks old, but if your choice is limited to a shy baby, or an adolescent who begs to sit on your shoulder, choose the older rat. Any rat this sociable will make an excellent companion. Males are larger than females when grown, and tend to be less active and therefore better lap pets. Both sexes make affectionate pets. Whatever sex you choose, it's best to get at least two.
Rats are very social and get lonely by themselves.
As long as you spend time with each one, they will bond to you as well as to each other. If you get just one rat you must spend a lot of time playing with him. Be sure roommates are the same sex, or have one or both neutered. Sexing rats is fairly easy with a few guidelines. You should be able to see the scrotum, though, which will take up much more room between the anus and genitals than in the females.