I don't mind if older people write to me. What bothers me is when someone doesn't put their real age on their profile. I don't care about age, but I care about honesty, and lying about something like that doesn't leave a good impression on me.
I understand that sometimes you just get a vibe from someone and you KNOW you shouldn't trust that person, and that's totally fine. But if you feel that way toward all adults, I'm not sure what to tell you except than change your availability settings.