It's random. They are supposed to have the same aging as the Sims in your house and if you move them in, they will. However, babysitters and newspaper carriers are notorious for aging up on the way over if you invite them for a visit. I've had a child sim in my house meet a newspaper carrier, invite them over that afternoon and the person age up to teenager on the way. Invite the same person over the next day and they were a Young Adult. The game just ages them up randomly but there are no specifics. I know one newspaper carrier that stayed a child for four generations but I also never invited him over.