Here's a fun fact for a Friday morning. Mario is apparently in his mid 20s, more precisely just "24 or 25" years old according to none other than Shigeru Miyamoto in a newly unearthed Japanese interview from 2005.
According to the interview, although Mario has evolved in lots of ways over the years since his inception, his age was set in stone from day one. In the words of Miyamoto-san:
I think it was fortuitous that we didn't put any restrictions on Mario as a character. Normally when you create a character and present him to the world, all the details get filed in: what's his favourite colour? what kind of food does he like to eat?
Aside from the fact that he's about 24-25 years old, we didn't define anything else.
Determining Mario's age was important so he could be used in later games and avoid the need to add a backstory at a later date:
When we make a game we take care not to add incongruence to that game's world. With that caveat, I'd like to keep using Mario in future games.
So there you have it, having facial hair might well make Mario look a bit older than his years, but it turns out he is a bit of a spring chicken - now and forever in the eyes of Mr Miyamoto.