The way I solved it in the past was that Elves don't live to be 100. A lucky human might live to be 75 or 80. With the assistance of magic he might even make to 100! An elf will normally make it to the 80s or 90s and Elven Mega Druids might be 150 to 200 years old, but they aren't concerned with the trees; only the forest as a whole so they still don't become ultra powerful world breakers. I usually run low-ish magic settings though so it might not work for you.