The last man over 40 years of age to win a Grand Prix was Nigel Mansell in Australia in 1994, when his younger team-mate had collided with Schumacher ahead.
Not counting the 1950s (there were an exceptional set of circumstances there), these are the oldest Grand Prix winners:
So of the 81 drivers who've won races since 1960, only 21 have won a race at the age of 36 or more.
It has an effect, there is a reason why sportsmen retire, in most sports, at that age!