Hmm ok. So like your saying that if the guy was 34 then it's fine? No risk of 'damaging' the girl and no 'responsibilty'? But from 35 onwards, older guys run the risk of permanently corrupting young girls and have to treat them as fragile objects, bearing all the responsibilty?
First, I think you are undermining her sexuality and ability to make choices to the extent of being patronising. Like she is some precious flower who needs to be carefully and gently 'unwrapped' by you, the older 'teacher'. That kind of smacks of a creepy 'kind daddy' act, instead of going into this as two consenting adults and being upfront. If its clear that after meeting her, the dynamic and vibe is not like two consenting adults, then its wrong. I guess where we differ is that I feel I can judge that from the offset. It just happens automatically in fact, as if it wasnt like that, the attraction wouldnt be there anyway. To me, your original post didnt sound like anything dodgy, it sounded like you have genuine rapport and chemistry, and she seems grown up enough to have a relationship, which she is actively looking for.
Of course there is an age where its wrong. But I feel that the law at 18 covers that well enough. From my two experiences with 19 year olds, and my memory of 18 year olds from my youth, I can't agree the 'default' assumption is that 18 year olds dont know what they want and need to wrapped up in cotton wool, which suddenly changes at 20 or something.
Secondly, in terms of your abilty to hurt or damage her, I cant see any difference to a younger guy. Are you saying that its ok for a guy in his 20s to fuck and dump her? Do you think its more damaging if an older guy mistreats her than a younger guy? Girls her age are horny, want fun, excitement and sex. Perfectly natural, and IMHO (for another discussion) its perfectly natural she would want it with someone older. And if she not doing it with an older guy, she would be with someone her own age, but still running the exact same risks of getting hurt, mistreated or whatever. (Arguably more so, given the immaturity of younger guys)
Sure she can get hurt, and so can you. My opinion its a matter of decency and sensitivity how you treat people, not so much magic numbers. If she is looking for a relationship with an older guy (for money!) then she also has to bear some responsility. Yes even at 18. Personally, I think suggesting otherwise is patronising.
Having said all that I do not actively go around looking for 18 year olds. Personally if im on a dating app 21 is my lower filter usually. But if I met someone younger and the mutual attraction was there I wouldnt back away provided it was legal (which was my only concern in the original post).