There are different issues with different ages. My DD was barely 3 when the douche left. She doesn't remember living with him. Her normal has always been going to visit dad on the designated days.
My DS was 7. He was the only child for a while and was extremely close to his father. He had a much harder time adjusting and still has a hard time sometimes.
I see different things that hit them at different times. As my DD gets older, I think she will resent that he left when she was so little and that he missed out on so much. He isn't exactly the most involved parent and she seems to get annoyed at times that she got jipped out of having a dad in the same house.
My DS is more afraid of abandonment because he remembers when his father moved out. He remembers him being here for birthdays and holidays and vacations. We never fought and we didn't live in a home that was overtly broken. In fact, I had no idea it was broken until it shattered all around me. It all came out of left field. DS feels the impact of that. He is more of the pleaser naturally anyway and is more afraid that if you make someone angry they might get angry enough to leave.
All of it breaks my heart for them. I had no personal experience with divorce in my family before I was forced to file for one so parents living separately and visitation schedules wasn't in my wheelhouse.
I can imagine that as a kid gets older and the roots of the nuclear family grow deeper, it would be that much harder to see your parents split. You may be more mature, but I don't think maturity lessens the pain. It's not like kids take some magic pill and are automatically able to handle all the shitty things life throws at them once they turn 18. In a lot of cases, I think it's worse. It takes a long time to process the grief and a long time to break habits and change traditions.
If people stay together just for the kids, it does nothing but delay the inevitable pain they are going to experience.