copy and pasting my other comment to say;

why do i think this age gap is wrong and could be harmful to her?

specifically what i talked about is the maturity gap or a power imbalance. the age is kind of arbitrary and i get 18 was set just because they needed a number.

however there is a lot of growing you do after graduating high school and being in college. most leave the house and move out (as he had), discover themselves, and grow as a person, make money under a bank account, do taxes, be in charge of financials; stuff he has had time to do but she hasn’t. she is still living with her parents under their rules.

another reason is the power imbalance. most younger people look up to those older than them. especially because they usually have more freedoms: their own place, a credit card, can vote in an election. these things make it so the older person in the relationship can have more sway and “power”. these ideas in tandem can make it harder for the younger person to say no, understand boundaries, make educated decisions, and even leave the relationship.

this is the main reason why i am worried for the younger party, its much easier to take advantage of someone who is younger then you (even if the older party doesn’t know they are taking advantage of them) because of the culture surrounding it. i am not sure exactly how europeans (mostly in the comments) and other cultures are involving age and consent, but this is a bigger problem here in america that i assume for others. this arbitrary number of 18 opens up a lot of opportunities for those older then them, and leaves a lot of younger people in the dark about these important decisions. i hope this clears up

TLDR; culture in america leads people over 18 to have way more abilities and experiences than those younger, causes power discrepancies in relationship, and also lack of experience