As best as I recall, the closest we've ever come to an official word on Mario's age is some 90's NOA interviews that pinned him as "middle-aged." I do think I remember NOE giving an age for Waluigi at some point, but I can't remember what it was.
But just for fun...
Daisy was around 21 in the SMB movie (the titles place her being dropped off at St. Teresa's 20 years before the movie, and she looked to be around 1 at the time), and was Luigi's love interest. According to XKCD's Standard Creepiness Rule (
http://imgs.xkcd.com/comics/dating_pools.png), that means Luigi must have been between the ages of 17.5 and 28 at the time. Assuming that Mario and Luigi are twins, as per Yoshi's Island, they obviously must be the same age. For Mario to look like Bob Hoskins did, their age must be toward the older end of the spectrum.
The SMB movie represents Mario and Luigi's first entrance to the Mushroom Kingdom. In current games, Mario and Luigi are very familiar with the kingdom and their reputation has become common knowledge virtually everywhere. They've probably been there for at least ten years, putting them around 35 - 40 right now. If the SMB movie were canon, that is. Of course, if it were, Bowser would be dead... though it's possible that Daisy's "You're not going to believe this" could have referred to Bowser somehow being revived in his current form, if they had ever gotten around to coming up with ideas for a sequel. But the movie's still not canon.