Well I obviously didn't read every title on MD but I guess that subjects of this theme would be so miniscule (like 50-100 titles out of 50k) so it doesn't worth the time and effort.
Edit: Moreover "age gap" term is very vague. How exactly would you define it? Is 1 year difference enough to tag the title? 5 years? 10? Where would you draw a line? And why X-1 is not good enough?