[Apollonian]

For one, it might simply be the difference between physical maturity and socially accepted adulthood. Maybe the initiation-into-adulthood rites require a century or so of preparation... (For example, a wood elf might have to plant and tend a tree into it's own maturity, thing sing a bow or staff out of its heartwood.)

For two, I've always had the impression that elven pregnancies are rare. Whether that's a natural difficulty of conception or the result of extremely long-lived and pretty durable people recognizing the possibility of overpopulation and having an effective contraceptive is up to the creator. (One idea: there are only so many elven should to go around, so a new will can't be born until about one dies. The elven God can create new souls but doesn't do so often.)