Originally Posted by Michael J Fox is Canadian
Well is it not logical that could have happened with Doc; He spends at least 10 years away from his timeline prior to returning around the end of part III and likely 10 more prior to the game (Jules is around college age) plus any of his travels alone with the delorean at the end of part I (we can only assume whether he went other places aside from 2015). If he's 18 in 1931 that means he should naturally be around 73 in 1986 but old doc has been alive for over 90 years so not hard to assume FCB doesn't live that long. You could also argue that because of how critical FCB is to that timeline, old doc had to be erased to make way for FCB (perhaps the same thing happens with Biff in 2015, that biff disappears but another rich one is there).
It shouldn't matter how big of a change it is. George beating up Biff and having a better life would have had a significant impact on Martys life. The least that could happen is his memory of these events would catch up with the timeline. Also I am pretty sure Biff faded because he got killed in the new timeline.
If his age did change then he would probably not fade out but be surprised that he looks younger.
Marty was expected to be somewhere in FCB timeline so why wouldn't he disappear into that Marty?