I think part of the "problem" is that Valentine simply feels older than 28. A relationship between a 28yo and a 20 or 21yo isnt even unusual imo, pretty normal stuff.On the one hand, I agree that Val obviously has more general life and sex experience than the 3 younger LIs. On the other hand, he only has slightly more experience than them in serious relationships and lags behind where other 28-year-olds might be in some regards due to his trauma etc. So I think arguments could be made for the age gap to be less of an issue than it might otherwise be.
But they feel way further apart, due to Valentine having all these life experiences, being very successful and wealthy, had to kill countless people, found and lost the love of his life, was close to suicide, while they are girls fresh out of a girls school.
Valentine feels more like he's 35 (same with els and mcnab) while amrit, vi and ines feel like they are fresh 18, neither of them ever saw a cock in their life lol