Hey, I just joined this discussion, so I don't know if this has been discussed, but I think I found a possible way to narrow down Dante's age.
Okay, so in DMC 5, we get the prologue scene where Morrison introduces V and Dante gets the phone call from Patty, the girl from the anime, about her 18th birthday party. Now, there is a line from the anime series where he said a line along the lines of "maybe in ten years, I'll ask you out on a date."
Now, some sources I've seen have varying accounts of Patty's age, having her at youngest 8 and oldest 13. Using this, we can guess a range of five to ten years between the events of the anime and DMC 5. Depending on the length of time between the events of DMC 1 and the anime, we can guess the range of Dante's age.
Taking an initial guess for the anime, I would say that at minimum, it takes place six months after DMC1, so Dante would be somewhere at 29-30. Using this, we can say Dante would be around 34 at minimum and 40 at maximum. Given his aged appearance, however, I'd guess he's in his late 40s at minimum unless a side effect of demonic parents is rapid aging.
I fully understand that this is not a surefire guessing method and the lack of confirmed details lowers its credibility, but I think it's a start at least to figuring out Dante's age. Feel free to correct me or fill in details, I'd love for a more accurate method to be found.