I see these questions pop up from time to time, but can't help but wonder if they're of any use.
All it seems to do is make anyone outside the 23-28 age range feel out of place. Isn't there enough worry already about being able to connect with new peers?
One of the things I love about grad school is that it's so transient. For some amount of time, you get to be surrounded by peers who are from different backgrounds, different knowledge bases, different sets of experiences, and so forth. With that, you get a number of students who don't fit the "standard" age range, but it also means that it just doesn't matter any more.
What does matter is that you can learn new information quickly, keep up with a hard-working lifestyle, and build relationships with new and interesting people. I don't see how age affects any of that, nor why anyone outside the "standard" age range should feel like they have something to prove.