According to a recently uncovered interview with Miyamoto from 2005, Mario's age was apparently confirmed!
I think it's very interesting that nobody really knew this until now. I really didn't think age was something they had thought about with Mario, so it's definitely odd to now know this. I figured he was closer to 30 but I guess I wasn't too far off!
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
I think it's very interesting that nobody really knew this until now. I really didn't think age was something they had thought about with Mario, so it's definitely odd to now know this. I figured he was closer to 30 but I guess I wasn't too far off!