It depends on a person's looks. Its the same with girls too. I've seeen girls who are in their late twenties and who look like teenagers. And I've seen girls who are in teens/early twenties look like they're in their late twenties/early thirties. It could be how a person takes care of themselves (managing weight, diet/eating healthy, wearing sunscreen, etc). It can also be how a person is naturally built; maybe they are broader boned, have a broader face, or their facial features are naturally more mature looking.
At the same time, I've also heard stories of people tampering with their age for marriage purposes. I guess you'd have to do an investigation/ask around (which is typically done in rishta scenarios) to find out if the prospect is really the age they claim to be.