You make it sound like a bad thing. At least someone is trying to ask you out. I seriously doubt some 40 year old is embarrassed or uncomfortable talking to a young lady even if you do not want his attention. So that leaves the younger set who just want to score. You are not a big sister or a young play thing so make that point very early in the conversations.
While I understand you want to be with guys closer to your own age, I do think you could consider someone younger - perhaps someone right out of college. After all, at most they are 9 years younger than you. While that may seem like a bit of a age difference, it's not really that bad.
Have you tried approaching and starting the conversation? If you're out and lets say having some drinks with some friends if you go and start the conversation first I'm sure you won't have much of a problem.