The Vietnam war ended in 1975. Born happens in 1971, when Frank is serving his third and final tour.
By then, I think he should be at least 25 years old, by the way he looked and acted. So in 2012, he should be 65.
I don't mind the age, since as a comic figure he could never age. What I think it'll be weird in a few years, is that Vietnam will had happened so long ago, The Punisher will feel outdated.
What do you think it's gonna happen in few years to the character? Do think they will give him a new war to be a veteran from?