at -16 the ratio is 8

at -8 the ratio is ?

at 0 the ratio is ?

at +8 the ratio is 3

so the ratio went down from 8 to 3 (5times) in 3 period of 8 year.

i.e. every 8 year is goes down by 5/3

so after one period of 8 years it was 24/3 - 5/3 = 19/3 or 6.33

so 8 years ago he was 6.33 years older than her.