Originally Posted by The Incredible Hulk
She'd be like 100 years old now. Steve's been in cryo-stasis for 70 years. I think it'd be a real stretch to have her still alive. Maybe just doing a scene of him visiting her grave would be more realistic. You could sort of get into the Superman type territory where the hero has to deal with everyone around him dying long before he does.
This is sort of the problem with Cap's origin. As time marches on, you've got to have him frozen for a longer period of time which means less and less people who knew him would realistically still be around. Sooner or later Cap is going to become like Demolition Man origin wise.
In May 2011 the Dep. of Veteran's Affairs estimated there were still 2,079,000 WWII vets out of 16,112,566 total US service members still alive. Further, they project that there will be 855,000 still alive in Sep. 2015 and 112,000 in 2023. (estimates midway down second page
So it's easily within the realm of believability that Peggy could still be alive.