26 November 2010 here!
@thibault was made on 22 November 2010, but the answer depends on how you define it.
When Thibault started working on it? That was on 3 June 2010, apparently, with this commit:
github.com/ornicar/lichess/commit/3950b46f5faa977724ab51c2b044894490acc803 -- but that's really when he first created a git repo.
When he purchased the domain?
I don't recall to the letter, but it was also in mid-late 2010.
So yeah, around 4 years.