I don't have any blanket issue with age different relationships. Love is great, and we should grab it where we find it. However as a man I must say this Steve - Diana dynamic as told in the comics was predatory, and any Gentleman would see it as such.
The age - life experience - difference between an average 18 year old (legal adult) and a 28 year old adult is far greater than that IMO than say between a 28 year old and a 48 year old.
Some young people are mature enough to make a decision to date much older people, but when an older person is placed/has in a position of trust, as a mentor or teacher - it is clearly inappropriate especially when the younger person is vulnerable.
I would ask people who defend such behaviour to take a long hard look at their moral compass.