younger...older... doesn't really matter...but for a relationship +/- 15 is my cutoff.
There is no minimum age ( other than legal, which I'll call 18). I'm 45, if I was 65, the minimum age is still 18.
The minus side of +/-, stops functioning at about 18, but you want 30, when you're 18, then when you're 30+, you want 18.
At 40, you want under 40, etc.
A relationship can be one night, those are often best !
For a +15 gap, you better be under 25, IMO !