If it hasn't already been mentioned here, in the Monster Manual, becoming a Half-Dragon increases lifespan by 50%. That alone pushes Elves to over a thousand years, Dwarves to almost 500 years, and Humans to 150ish. Gosh, a Half-Dragon Elven Druid could live for [(750x1.5)x10]= 11,250 years!

Also, you could always Plane Shift to the Astral Plane and simply stop aging. Plenty of work to do out there with only brief visits back to the material. No need to cut off a pound of your own flesh each time you wake back up in your tank.

I love it. :D

My favorite explanation in High Fantasy: "Nobody has no idea how it happened" 

It may seem lazy at first glance, but really it's one of the best DM tools ever. It avoids going down unnecessary and time-consuming rabbit holes, it creates mystery, it creates possibilities etc. In fact, the less you explain in detail, the more possibilities you leave open for future lore.

I have an active character who is also who-knows-how-old. A druid/life cleric. He is a Tree Spirit (fey) by race. He originated from the Tree of Life and has a sort of Gandalf-like BG where the tree of life sent one of its spirits into the world. My character was chosen for this task and took the form a gnome.

Now he is fully a gnome biologically and he is very old and frail. So at least some hundred years old. He was defeated by the main villains of the story when he was in his prime. He was a remarkably powerful druid, but we never determined how powerful he was at his strongest. He was "petrified" (into iron) by the evil spirits but shielded parts of his mind with a magical mind palace. Lost much of his past while petrified, but kept his sanity and the most important bits of his Arcane knowledge etc. 

A) we don't know how long he was petrified. He was freed in the campaign.

B) we don't know how old he was before he was petrified or how long he was petrified. The DM has given hints in-game, that he has encountered these evil spirits many times throughout the ages. The DM even hinted that this may be one of his many reincarnations and that he has sometimes defeated the spirits and sometimes lost. I get the feeling that this is sort of an endless cycle of good and evil, which is really cool. 

C) we don't know how powerful he was back in the day, so he may have lived a lot longer than a typical gnome. 

In conclusion. We have no idea how old he is. :P

He is now level 6 after spending "eternity" as an iron statue. But his powers regrow fast. 

We worked together on this with the DM. I wanted a character who is old and frail and has that age-old wisdom and has most likely been in lots of places. DM only wanted to create the locations where we currently are and keep world building at minimum. So this was a beautiful compromise. DM drops me amazing flavor content like "you remember this place. You remember that corridor. You have been here before."

 Edit: Oooh hey I forgot, this character is the one in my profile picture. 

Quote from Blueninja33 >>

If it hasn't already been mentioned here, in the Monster Manual, becoming a Half-Dragon increases lifespan by 50%. That alone pushes Elves to over a thousand years, Dwarves to almost 500 years, and Humans to 150ish. Gosh, a Half-Dragon Elven Druid could live for [(750x1.5)x10]= 11,250 years!

Also, you could always Plane Shift to the Astral Plane and simply stop aging. Plenty of work to do out there with only brief visits back to the material. No need to cut off a pound of your own flesh each time you wake back up in your tank.

The Half-Dragon thing is new to me! Thanks. I need to look into it. :)