I think it mentioned in earlier chapters (maybe chapter 1 or 2, book 1) when Lucia proposed to Hugh, just like Slither said : at that time, Lucia had just turned 18 and Hugh was 23, so it’s a 5-year gap.
About Damian, it’s mentioned in some chapter (forgot which ones) that Lucia & some people who knew about the father-son relationship between Hugh & Damian thought that Hugh was too young to have a son of Damian’s age.
Hugo (Damian’s biological father & Hugh’s twin) was about 15 y.o when he fathered Damian (without him knowing) & doctor Philip brought toddler Damian (about 3 y.o) to Hugh after the death of Taran’s family members (the former Duke/Hugh & Hugo’s dad, the former Duchess, and Hugo himself who commited murder-suicide) when Hugh was about 18 y.o

But dat soo cute lovely shi AAGH