[Polkageist]

It's a little like Benjamin Button, but consider that elves may flip the script when it comes to emotional maturity and effects of puberty.

For such a long lived race, risk aversion especially early on when learning how the world works may be really valuable. Young elves' bodies may 'turn down' emotional responses to things like social slights and risk taking behavior. As they grow, their impulses push them to be cautious and considerate. To a human, a young elf may appear to be very controlled and mature as they have a very serious attitude towards new situations that need careful evaluation. Their brain chemistry encourages focus. That would give a young elf a huge advantage in safely learning all the tricks and skills necessary to live a long long life.

As they age, those mechanisms break down as the older elf no longer produces enough moderating chemicals to keep their minds focused and their emotions in check. There's that one epsiode of Star Trek TNG where Sarek is so old that he's just not able to keep his Vulcan self-control going and has outbursts of emotions and irrational behavior. An old elf could become very passionate, creative, and moody. Grandpa has decided that he wants to become a musician, and sow his wild oats!

Maybe that's why elves don't have kids running around all the time. They don't WANT to have kids until they're much much older and are out of their young self-controlled phase.