You can partner hamsters whenever you want, whatever their age, but only if they get along. To see what hamsters get along with each other, read the care book at the pet shop.
When your hamster is 21 days old (3 weeks), it is an adult. At this age, female hamsters can have babies. To get your female hamster to have a baby, make the 2 partnered hamsters play together a lot (over night and all day with your DS on charge usually works). Once your Female hamster is pregnant, the shop owner will tell you. All you need to do now is wait 10 days. If you Cant wait that long, you can get your hamster sooner by just changing the date on your DS settings.
As soon as you play the game after 10 days, a message will pop up telling you that the baby has been born. If you have a spare cage, it will go there, but if you don't, it will go to the care centre. If both the care centre and the 3 cages are full, then your female hamster wont become pregnant.