On the question How could Aragorn be 88 years old while he looks relatively young? there is the following comment :

One tiny bit of trivia: at that point in the books, he's actually 88 years old. His birthday was the day that they met Gandalf the White in Fangorn forest.

- Plutor

Then :

How do you know his birthday?

– Earendil 17 mins ago

Is there a quote or a something to support Plutor's affirmation?