If I were single I’d probably be looking 10 years either way, but that said the vast majority of girls I’ve dated have been younger than me, but usually only by a matter of a couple of years, the youngest was 6 years younger than me (amusingly she’s now married to a guy six years younger than her).
Not long before I met my current girlfriend I had a brief relationship with a woman who was a fair bit older than me, almost 14 years. It is amusing because she went on about almost being old enough to be my mum, and I had to point out that my mum was quite old when she had me, and that my own sister is 15 years older than me so strictly speaking she was just old enough to be my sister. I do quite like older women, I think I just tended to rule them out purely on the basis that I didn’t want to rule out having children someday, which was probably a mistake.
On the whole I think it would be hard to draw an arbitrary line because you never know who you’ll fall in love with, but someone 15/20 years younger than me and I’d worry what we’d have in common, and someone equally older than me and I’d worry how long we’d have together.