how old and telling what sex a rat is

Question
I just bought a baby rat today from a pet store. There was baby a couple different sizes. no adults. When I got home and was hold "her" I hope her. she was one of the littlest ones. 3 & 3/4" to 4" long stretched out tip of nose to tip of butt. She was seeming to be obsesitly licking my hand "as if she was tring to nurse" She did seem to intrested in a pretzle or pizza crust I tried to give her. She did take them but just put them back down. I bought "lab blocks"what they were feeding her hen she was in the cage by herself she moved the two I put in along with the pizza crust , preztle before I went to bed I crushed a block and add a little warm water which she seemed to really drink..  is there a way I can tell how old she is and what sex she is? She is really light in color (white & cream) and when are they old enought to leave mommy for good?

Answer
Babies leave Mom at 5 weeks old. Some breeders wean as early as 4 weeks but I prefer to leave them in until 5. Some babies seem abnormally small for their age and appear younger than 5 weeks; some are right on schedule regarding growth.. a 4 inch baby is usually right about weaning age. At weaning, you should be able to flip your baby onto its back and see two testicles if its a boy. They will also have a protruding bump spaced about thumbs width away from their anus, this being the penis. A girl will have a smaller bump much closer to her anus and obviously no testicles.