Rudimentary math time! Okay, so this took a little digging (and now I feel kinda gross), but here's what I found out about the Disney family lifespan.
Walt's immediate family - 11 people that I could find (2 parents, 4 siblings, 2 daughters, 1 wife, 1 nephew, plus 1 Walt to keep things semi-accurate).
Average life span of those 11 people, including the ones who are still alive (Roy E. and Diane) =
79.06 years (give or take a few tenths). Add that to Walt's birthdate of December 5, 1901 and my guess is:
December 15, 1980. Oooh, weird!
Now, this is taking into consideration his mother (who died of unnatural causes), his wife (who he wasn't related to - um, let's hope), and his daughter Sharon (who was adopted). So needless to say, my math's a little fuzzy. But I think it's close. And close counts in hand-grenades, so that's saying somethin'.
Oh yeah, and here's one more thing. The average life expectancy of the typical person born in 1901? 49 years. So Walt actually outlived the average guy in his age group by a good 17 years.