How big is too big of an age gap?

Like the older you get the less it matters. Say me whos 31 years old dating someone who is 19. Thats suspicious, questionable especially if you dont know the two involved. But like 10 years down the road, 41 and 29 doesnt seem as big a deal right? But if the genders were reversed, say an older woman and a younger man it seems more innocent and acceptable. My sis is dating a man 10 or 11 years older, no one batted an eye. Im sure there will be some polar opposite opinions on this thread.