i remember when i got my very first rats from petco, when i was 13, i think (almost eight years of rats in my life. wow.) i wasn't very knowledgeable then, i'm afraid, and i guess i picked out a girl and a boy (both from the tank labeled "female"- so i guess the staff wasn't very knowledgeable, either, come to think of it...) we got all the way home before we realized (the boy must have been scared or something), and my mom wouldn't let me keep both. (in retrospect, i realize that could have been a disaster, had i adopted a pregnant rat then. i'd have had no idea what to do.) i felt terrible for having to "return" him and get a girl instead... i suppose i wasn't too traumatized, though, since i've had many, many rats since.
it's hard to pinpoint exactly when i fell in love with rats.