1. I hate this hole idea of not replying at all if you're not interested "to avoid offending" or other anything similar. I for one would rather just get a reply saying "sorry. not interested." and maybe a reason why. But at least say when you're not interested say something. Don't just leave it to "take a hint" or "get real".
2. If you appear desperate on your profile then you very well might be so you should likely not have your age setting precluding people under a certain age. I've seen many womens profiles who are much older than me who i would gladly have sex with. that goes for couples too. I'm not saying everyone should accept all ages but be open to it.