What is it with these arbitrary ages? What if one is 32 and 37? Where do you draw the line?
Lack of responsibility does not necessarily guarantee moral clarity. For example: a crack dealer is not responsible for a customer's health. However, dealing crack is morally reprehensible.
Sorry, but you aren't really making a case, here. While it may not be objectively immoral (I do think the existence of objective morality is provable) to have casual sex, I have yet to be presented with a situation where it isn't, at the very least, inadvisable.
You know, I don't mind sharing the seat...