I wouldn't be happy at all if 26y man was trying to get with my 18y DD.
Yes they are both technically adults but are general worlds apart in terms of life experiences at that age. For that reason it is inappropriate, In a few years time that kind of age gap wouldn't seem so bad - but when is just 18y its just too big.
DD is only just out of school, still dependent on us, her parents, financially (and often emotionally, etc too) and has yet to experience a lot in life. She still has years of education ahead of her, still to experience living away from home, etc.
You would expect a 26y to have been in work for a few years now, living independently, financially independent and to have experienced a fair bit more of life.
Many 18y think they are grown up. You only have to be in their company a short while, listening to them chatting, to realise that they aren't really yet fully grown up at all. In my years of teaching and knowing many 18y I have yet to meet a fully independent, fully grown up one.