You're an adult now... I think you're just not used to the expansive age gap in guys who can now 'legally' hit on you. You're young, naive, and therefore an easy target.
I would suggest you get used to this... learn how to politely refuse (and be more forceful if the situation warrants it)... and carry on with life. Refusing a 40 something man is no different than refusing a 19 year old 'boy'... as they are both 'adults' exercising their legal right to pursue you.
Isn't being an adult great?
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen