If you go looking for something, you will find it.

What I mean is that you are clearly searching out these doubts that you have. And you are SPECIFICALLY looking for confirmation of your assumptions about this age/appropriateness thing.
Just like if I Google "why men don't like short women"...then yeah. I will find a gazillion affirmative responses to that....

1) Any girl over 18 is okay to ask out.
2) You might get turned down.
3) You will never know the reason they turned you down.

You can cherry pick stuff off the internet to validate your reasoning, but I'm telling you that your research method is waaayyyy flawed.