First off, what mental maturity?
On topic though, if you're referring to pre retcon, dead at 20 lifespans, then i'm tapping out, we simply don't have the kind of evidence to know when they reached certain milestones. And to be perfectly honest I don't think Tom had it all planned out.
Post retcon though, as far as the sexual side goes, I just can't reconcile Flora for example as being an adult that long, she says she was a virgin before trace, and Keidran go into oestrus 2x per year, if we assume her first is at age three that means she's had at least 16-17 heats, and I don't think she has the personality/willpower to resist her urges that long. (Okay not actually in heat at that festival, but under aphrodisiacs so I feel the logic can still be applied.)
Anyway, that Maeve chart has her looking pretty obviously like a toddler, can barely walk on her own, it seems. I can't imagine them going into heat at that age.
Intelligence wise, I believe it is something similar to your first theory, except that the faster learning ceases at about the time of adulthood, 8. Let's say they were learning at approximately 2x the rate a human would (even faster the younger they are, gradually declining over time) and then from adulthood onward they are equal to a human. Let's assume an 8y/o Keidran is equivalent to a 16y/o human, that makes a 10 year old like Kathrin equivalent to an 18 year old. I think that is a fair comparison, even if Kat is a rather naive/ditzy example.
As for why the Templar are outsmarting them/Keidran seem dumber than humans, I think it's safe to assume that Keidran don't have much (any?) of an education system going on, and spend at least as much time fighting/raiding each other than attacking humans, in fact wolves seem to be the only keidran showing any kind of aggression to humanity, and they still find time to send slaving parties into fox territory, at least. Whereas the templar are putting all of their focus on eradicating/breaking one tribe at a time, currently it's the wolves.
Food for thought though, some dogs (decent sized dogs with decent sized brains) can be taught to ''read'' words and simple sentences at a fraction of the age that our children can. And of course they don't know what those words mean but they can differentiate between them and associate them with various things. E.g. ''If I touch the card with this sentence written on it, Dad will give me a treat.'' Yet dogs are of course far less intelligent than us when both are fully grown.
So I don't think it's outside the realms of possibility that an intelligent species might learn faster than us up to a point then become level with us.