8 years. In the last chapter of the second volume I believe they show the ages (18 & 26)
Is it excusable since khalid is very mature?
In my opinion: No. Considering that despite the forced marriages of minors and their distaste towards their innocence being robbed, it was still normalized in actual history and in some countries today. So aside from irl situations, this age gap being so wrong, I guess you can just conclude for this manga ....it is fiction.
Sort of in the sense of: just because you enjoy reading thrillers with psychotic serial killers, does not mean you have to like the act of a human etc. being unalived irl.
how much is the age gap between them again?