If we really really streeeeeetch our suspension of disbelief, then at best Howard is a cradle-robbing so and so in order to BOTH be around for Captain America's origin AND be Tony Stark's father. Do you think "Marvel's Agent Carter" might tinker with his timeline to close that gap or explain it away?
Actually, there's nothing that needs a retcon to explain. If we assume that Stark is 25 during Agent Carter in 1946, that would mean he had Tony when he was approximately 55. Even give or take a few years, a man having a son while in his 50's is easily within the realm of belief, and matches up with the films for the following reasons:
Being born in the early-to-mid 70's would then place Tony at approx. 40, which matches up with Tony's apparent age during the films.
Howard Stark having Tony in his 50's matches up with the appearance of the elder Stark in "Iron Man 2", shown when Tony was still a small boy.
His advanced age explains why Howard Stark died when Tony Stark was still in his late teens, as mentioned in the "Iron Man" films. After all, by the time Tony reached 20, his dad would have been almost 80.
No mention of Tony's mother is made in the movies, but given that women rarely give birth past their 40's we can assume that Mrs. Stark was at least a decade younger than Howard at the time Tony was born.