You are trying to read the minds of the entire population. Do you notice how everyone here has said that it doesn't matter? Once someone is out of highschool it doesn't make a difference. If you want to have a better judge of when people MIGHT start to look at it weird.... judge it by appearance. If you look like you could be someone's Father or Grandfather then it is perhaps too big of a difference.

People on this forum come from many different areas around the world, and are from many different walks of life, and many different generations. It is a good representation of this society that you claim to understand completely - and yet no one here cares.

The only people it matters to are you, and the people you date. And you cannot ever know if someone has an issue with the age difference unless you get to know them and ask them out. Plain and simple.