Kittens - how old when start purring?

Is learning to purr part of maturing for a kitten? If so, is there some particular age when they might be expected to start? We picked up our 2 Maine Coon kittens last weekend, and by now they are comfortable with their new home and family. The little girl, the younger of the two by about a week (She's about 8 1/2 weeks right now), adapted the fastest, and is sleeping on my toes at the moment. The boy, a week older, has a real ready purr, but the girl hasn't, to my detection, purred yet, even when she jumps up in my lap for petting.

Any information available?

I guess that is up to the individual cat, for instance my Loki purrs but very low and sometimes I have to but my hand on his throat to confirm the purr. On the otherhand there is my mom's girl Cookie - you can hear her in the other room, she purrs like a truck driver. Give her time and even if she dosen't no problem.

I have some abandoned kittens that I have had since they were about a week old.I really can't remember when they first started purring,but they are now about 5 weeks old and purr all the time.