Patrick Stewart and Rene Auberjonois are the oldest, being born in 1940. 78 isn't too bad.
Eh, it's about the average male life expectancy. Though given "retired" Trek actors are mostly modestly wealthy to comfortable, they should have a few years on that as well.
Still, people die in middle age of things like heart attacks and cancer all the time. The TNG/DS9/VOY/ENT casts have been remarkably healthy overall. Look at Babylon 5. O'Hare, Doyle, Biggs, Furst, Conaway, and Katsulas are all dead already - basically 1/3rd of the main cast.