I've found that by 12 weeks they are old enough but the pelt won't be full size, (we butcher anywhere from 5-7 months) after about 6 months the pelts will be full size.
An acquaintance of mine that tans leather waits until after their first molt - he wants to make sure the "baby" fur is gone and the rabbit's adult coat has grown in. So he waits until they are a few months old at least.
This came from rise and shine rabbitry's website on how to tan rabbit pelts
Rabbit pelts vary widely in quality and value. The different types of fur characteristics vary depending on the breed type, such as the Rex, Satins, Silver Foxes and other fur type breeds(also know as dual purpose rabbits- meat and fur) Pelts from the young of any breed are normally of poor quality and of less value than those from adult animals. The young fryers pelts are thin and can tear, but are still usable but the older rabbits make a stronger finished product. Those with dense winter coat that is not easily removed from the pelt, are most the most sought after.
Don't know if you needed the how to info but wanted to put it in here for people who do searches on this topic in the future. I found them very informative and have them bookmarked for the time when I am ready to try it. PO