5 yr age difference when you're 16 is major. 5 yr age difference when you're in 40's, pretty irrelevant. Rule of thumb, back in the day was; older partner, half the age, and add 7 yrs, is socially acceptable youngest partner. EG...
20 yr old with no younger than a 17 yr old, 40yr old with no younger than a 27 yr old, 60 yr old with no younger than a 37 yr old etc. Anything outside that rule of thumb, just, to many, seems suspect or a little creepy. Signs of manipulation, or daddy/mummy issues.