How old is Romeo? remains the most popular post on this site, by a long shot. Every day I land hundreds upon hundreds of people looking for the answer to this question. (The answer, by the way, is “Despite saying that Juliet is 13, Shakespeare never specifically says how old Romeo is so it’s up to your own interpretation.”)
How old is Hamlet? is also a big search result. He’s either 30 or 16, depending on how you prefer to interpret the gravedigger’s speech. I think the evidence is stronger for 30, myself, but I don’t think the character behaves like a 30yr old.
Pick another one. For which other character in which other play do you think that pinning down an exact age is a big part of understanding that character? I saw an As You Like It once where I felt that Rosalind, an otherwise strong character who ties the entire play together, was reduced to (literally) a giggling school girl who was too much with the “OMG! Orlando looked at me!!!” stuff. I never thought of her as *that* young, but I never really tried to pin an age on her, either. Late teens, early 20’s?
(For the pedants out there I’m obviously not trying to an *exact* age. I don’t think that it matters if Romeo is 18 or 19, but I do think there’s a difference if he’s 13 or 18. I don’t worry about whether Polonius is 65 or 66, but you could probably make a case for different behaviors depending on whether he’s 55 or 85. Like that.)