Both 13th and 14th have multiple wives... So they are definitely not single! XD. People generally marry young back then and as princes you really have no say in regards to your marriages, especially your first wife. Also, when Ruo Xi goes back in time, 8th was around 22-23 and 4th was 26, so the age gap isn't that bad. It's as the story progresses do they age to 40s while Ruo Xi ages to 30s.
This is why I actually support Tangren for hiring older actors. They fit historical facts and have the maturity needed for the characters better than younger actors hired for only looks.