According to Miyamoto, the only concrete detail they settled on was that he's about 24 or 25 years old.
"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. 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."
What the hell. I'm almost the same age as Mario and my moustache is not even half as awesome.. :/
Link: https://www.gamnesia.com/news/shigeru-miyamoto-says-mario-is-only-24-or-25-years-old
---Member of the official Squeezol Fanclub---