Legolas has no idea how old he is. None.
He has a general, ball park idea of his age, he can (usually) guess it within 20- 30 or so years. But that’s all.
He lost count somehwere in his first thousandth years of life and never bothered to figure it out again.
However, if you ask him his age he will tell you WILDLY different answers that seem to be randomly selected out of thin air.
Pippin: Legolas, how old are you?
Legolas: I am 8987 years.
Aragorn: Okay no, Ada isn’t even 8000 years old yet. Try again.
Legolas: I am 72 years old.
Aragorn: And you certainly arnt younger than me.
Legolas: I am 678 years old.
Aragorn: I also feel like that is incorrect.
Legolas: I am 3000 years old.
Aragorn: Much more plausible, but still, I think, incorrect.
Legolas: I am exactly 8 years old.
Aragorn: why are you the way that you are
Elves actually count time in a period called a yén iirc, equivalent to 144 of our years.
So it’s not that Legolas doesn’t know how old he is. He just wants to put things in measurements his friends can understand, but he’s very bad at math.