An 18 year old hasn't even developed their frontal lobe yet, they SHOULD NOT be marrying (or getting into relationships with) someone 10 years their senior. Age gaps are fine, but only when both parties are older, not when one is barely an adult.
Also, 14 yr olds weren't marrying old men as often as you think. The main reason why everyone thinks this is the case is bcs the main ppl (old men) marrying little girls were royals, meaning that it was basically given a pass. I'm not saying commoners and other nobles didn't do so, I'm just saying that it mostly the royal family who were kiddy diddlers.
Also, just bcs it was okay back then doesn't mean it should be written into the story. It's fictional writing for goodness sake, it's okay to make it historically inaccurate.
Ps: sorry if this came off kinda passive aggressive, that wasn't my intention at all
It also depends on culture.
In Western societies, what was actually happening in the teen years was courting. They were usually chaperoned to preserve the woman's honor.
Marriage usually took place sometime in their 20s. The average age gap was around five years.
Men had to spend more time establishing themselves and setting up a household before he could bring in a wife to manage it and fill it with children.
A ten year age gap might suggest a deceased wife or a man away on crusade before marriage.
So, yeah, if this is emulating western culture, it's weird.
Guys, that's not some huge gap, let's be real. At that time it was normal for 14yo girls to marry 45yo men.