I liked it, I think you have to remember it’s fiction. In real life that would be really bad, but in this story he truly loves her and is very self conscious about his age and making her happy.
Yes he probably should have waited until she was 18 but as this story is set in medical times she was probably deemed old enough by societies settings.
My issue was the “king” he literally assaulted a 14 year old 10 seconds after meeting her. Also the nobles who wanted to sleep with her to regain their youth. Like I think you have to realize that if her husband wasn’t there, she would have been passed around and raped the minute they found out about her powers. Her marrying him was the best scenario and she’s happy, that’s all that matter.