I have daddy issues, so I better not speak based on my personal experience 🙂

I’d say about seven years, based on a feeling. It’s not really linear though, is it? When young and when old, the gap should be smaller, but in your thirties, for example, it could be much wider and not be problematic. Then again, having a small gap is no guarantee of anything. People still sometimes get ill or die sooner than expected. Then there’s the issue of having children, which may or may not be a factor regardless.

Realistically though, you’re not going to fall in love based on age 🙂 Though if you do, and you keep falling for significantly younger or significantly older, it might be a good idea to look into “why?” Generally speaking. Not aimed at you Big Blue 🙂