Briefly: they don't have ages, so make it up! (I personally have Clint born in '69, Natasha '77, and Avengers happening in 2011 for various reasons)

They've been in the comics for decades, so they are...in a lot of ways, they are ageless (although I think comic!book Natasha was born in 1927, and when the Red Room got hold of her they did experiments on her so she's a lot like Steve - but other than an AU!idea where I want to poke at her being a freedom fighter at the age of 15 in the ruins of Stalingrad, I ignore this because I like her being normal!human.)

Also, I think you can pick and choose how much of comics!canon you want to have, because comics and movies are really happening in two very separate universes.