Ielenia's write up feels accurate. Anyway, it seems the majority of us who play Elves agree that 80 = 18.
I've been doing some math... It's hard to divide 80 by 18, but if you round it up to 90, 90 divided by 18 comes to 5. So we now can see a formula for the process of Elven maturity... Five Human Years equals One Elven Year of growth. That means that at age 100 they'd be '20 years old' and at 105, "21", etc...
But then we'd assume, at this point, that their growing stops or plateaus as the Elf comes into maturity, and will not show wrinkles or signs of aging until well past 1000 years (as per Ielenia's chart).
In Tolkien, however, the Elves never showed any of the afflictions of mortal aging. After thousands of years had passed, some of them would become so distraught at the changing of Middle Earth (or having fallen in love with a human who died long ago), that they'd choose to go to a favorite grove or mountain and quietly let their spirit return to Elvenhome... no trace of the body left behind. They were truly at one with nature (Even the 'High' Elves or Grey Elves, as Sylvans were considered almost 'Dark' in nature due to their breaking off from the Elven 'family' and taken to the deep places of the forest, and they were often Hermits) and could meld in between both spirit world and physical one - perhaps the secret of their longevity?
The Drow or Moriel aspect of 'Dark Elves' is a new thing entirely - Tolkien never wrote about that so I assume it comes from another game or is someone else's brainchild... And a brilliant one, at that! Personally, Lindor kinda has a thing for them.
So it'd be hard for me to have an opinion on them, anyway, as I've never roleplayed one and I'm sure I wouldn't be good at it, though I admire those who do and do it well.