I've edited this, as I posted it before I realized that Derren had been asked not to post. Hopefully this is ok. I've attempted to make this somewhat neutral, focusing on discussions (by several parties) of elven reproduction rates, without focusing on his comments.
I think something that everyone can agree that the idea of a race taking 100 years to generate a grown adult would definitely have their backs against the wall. Yet, the rules don't really say this. All the PHB really says is that adventurers typically start their careers around 100+ years. It doesn't say anything about their maturity levels before that.
So, this morning, I decided to look in the closest source to core that we have....Masters of the Wild.
Now, I'll admit that I did find in the book that it mentions that elves are less fertile than humans, and that a typical elven couple might have 1-4 children over 50 years, whereas typical human couples have 1-4 over 10 years.
Of course, there have also been spells in the rules that increase fertility...both in 2nd And 3rd Eds. they caused the target to conceive or impregnate (as appropriate based on the gender of the subject) with 100% reliability during their next "encounter". If the elves were seriously pressured, I could definitely see them using that magic, which was very low level.....like lvl 1 or 2 spells....so, accessible to a large number of their spellcasters. I will 100% admit that these spells are not core. They were in Green Ronin's Witch's book, or Relics & Rituals, or something like that. And in one of those big religion books for FR in 2nd Ed. I'll need more time to find the specific reference. The point is that elves have a fair number of spellcasters, and if they had just gone through a war, I could see them using magic to create a "baby boom". They're individualistic, but they're not stupid....and it's mentioned that they value children highly.
Anyways, Masters of the Wild states on pg. 13 that "Elf children grow almost as swiftly as human children to age 15 or so: a 10-year old elf boy and a 10-year old human boy are nearly the same size and have similar mental and emotional maturity.". It then states that humans finish filling out between 15 and 20 years of age, whereas elves take until 25. The delay to 100 is a cultural delay....not a physical, emotional, or intellectual one. Similarly to how in North American cultures, they say now that 30 is the new 20. In our advanced culture, people are often remaining somewhat dependent until their late 30's. But it doesn't mean that they're physically or emotionally incapable of surviving on their own. Masters of the Wild mentions that elves are discouraged from marrying before 100, though they're perfectly capable of doing so. It further mentions that when elven communities are under pressure, the delay to 100 is bypassed, and you have 25-year old elves, who are completely adult, taking part in war, etc. as necessary. They are not child soldiers at that point, as elves are fully grown in all ways by 25.
Further, it states that they are raised communally by their people, after the age of 10, which means that they're far less a drain on any individual family than even human children are on their own families.
This is very very different from the idea of them taking 100 years to reach adulthood.
It's perfectly possible to have an adult elven adventurer at age 30. The impression given is that the elves spend more time training their people...they're investing more in their "human capital" than humans do. I think the people who are suggesting the demographic charts in the DMG are likely not really applicable to elves might be correct. For all we know, PC elves tend to start at lvl 1 at around 100 years old. But maybe there are many 100 year old Commoner 2/Fighter 2's walking around in elven villages. They gained training and experience between ages 25 and 100, but, given they're NPCs, it's unlikely that they'll progress as rapidly as the elven PC, who starts at lvl 1 (to balance against other PCs), but will rapidly outstrip the abilities of the elven villagers from his home.
Banshee