Let's hope I can finally get the forum to cooperate to let me post!
I have bred lionheads for almost 3 years. When I first started, my lionheads weren't the best "quality" ...especially in the mane gene. I'd frequently get judge's comments of "great type" and "nice fur" but "needs more mane". In those litters, I frequently saw the mane start to "bloom" around 6 weeks of age to 2 months.
I made it a point to bring in lionheads that had better genes for the manes (by looking at the parents, etc). Now - by about the age of 3 or 4 weeks, I can see the mane start to bloom....its hard to explain - but you can see a difference in the mane and the rest of the fur. By 5 weeks now you can usually tell their lionheads by their mane and by 6 weeks its starting to look really nice.
What is interesting to me is that my does usually keep their manes - my bucks don't tend to keep them as much - BUT - some of my later bucks have kept their manes into adulthood better. I don't know what I find that so interesting - but I do.
BTW, you can usually tell if a lionhead is a single or double mane within a couple of days after birth....as their fur is coming in. After that - as they get older - the only real way to tell is by the mane itself. I find that single manes are often wispy - but that is not always so. I have some single maned animals that have the really crimped look to their mane and it keeps coming in fuller and fuller.
I hope this helps in some way - and I hope the forum will let me post this!