There's been a few uses that I've seen:
1) They just have evenly slow rate of growth. (Eg. 40years = 4 year old.)
2) They have a "growth spurt" where they age roughly at human rate until the peak of their physical condition (20s to mid-20s) and then kinda stop growing.
3) They are technically spirits and therefore technically eternal, so they kinda pop out the way they look.
So... Yeah, just do whatever you want in your own world settings.
Modern Elves are pretty much a Tolkien thing anyhow and not even like the old Elves in traditional folklore, so there's no actual tradition to follow (except commercial traditions ie. The ones in popular media. So, Hentai Elves? Why not. =w=b )
Earlier today I was just suddenly thinking about how people nowadays have pretty much forgotten how much Celtic Druids were into human sacrifices back in the days. Like, Really, REALLY into human sacrifices...
That's also kinda part of our commercialized traditions now.[/edit]