THE MATH OF DANTE'S AGE
Dante is supposedly 19 in DMC3.
DMC3 manga is set 1 year before DMC3. (Dante= 18)
DMC3 manga implies Eva was killed 10 years before the manga (18-10= 8 years old when Eva died)
DMC1 is 20 years after Eva dies (8+20= DMC1 Dante is around 28)
So unless DMC4 is one year or less after DMC1, Dante is indeed older than 29. According to Kobayashi, the games director, Dante is approaching 40 in DMC4, which would make DMC4 more like 5-10 years after DMC1. Since he intended Dante to be almost 40 he likely instructed the artists to portray him that way, hence he appears in t he range you described. He didn't however think that through, because he said Lady is in her 20's in DMC4, and if you played DMC3, you'll see Lady isn't that much younger than Dante. It is possible she is in her late 20's and Dante is "approaching" 40 at a bigger distance than you would assume, but it's still rather sketchy.
Quote taken from this thread:
http://boards.ign.com/devil_may_cry/b5258/175111560/p1/
What. The. HELL?! So, Dante ages faster than humans then? Lady was a year, maybe two years, younger than him in DMC3, period. She looked damn young to me in DMC4. So how come she can be in her 20s and Dante close to his forties, if they were BOTH kids in DMC3?
I don't get it. It hurts my head trying to figure it out. Sparda was talking about logic. WHAT LOGIC?!