I hear this a lot from people. I think that's true but only to a degree. For instance, I'm 33 and I have friends who are much older than me (one good friend is almost 70). In that case I do think it's just a number, though I recognize that my older friends have been through a lot more of life than I am and I respect them for their wisdom. But we're both out there supporting ourselves and have the same sorts of responsibilities (paying bills, going to work, etc.).
On the other hand, I heard that this morning from someone who was 15 and wrote to me despite knowing I was 33. In that case, I think it's NOT just a number. A 15-year-old and a 33-year-old are in SUCH different places in life and I just won't chat with someone who is so much younger than me. I just don't see us as having any sort of common ground.