More people are living at home for longer, because it's hellishly expensive to move out. Living at home doesn't mean you have no ambition, drive, independence or worth. Often it means you're still studying, you're saving money, and/or you have a good relationship with your family.
There's a big difference between someone living at home who's working, contributing to the running of the house (both financially and physically, with chores and such) and is actually an independent person, and someone who's slacking off, unemployed, not studying, staying home to play video games and expects others to clean up after them. The first, I'd date. The second, I wouldn't. It's not about where you live, it's about how you do it.
A lot of guys in that age bracket aren't emotionally independent. They don't know how to take care of themselves or their environment. It's not a turn on to go into a house that's never been cleaned and is covered in pizza boxes and the rubbish bin has become an entity that's asking for voting rights. For those guys stuck in the party mode, they don't exhibit great relationship signs.
It's perfectly reasonable for someone to reject advances from anyone, at any time. No one is owed anything.
Oh yes, the 'nice guy' dilemma. These aren't nice guys - they're entitled guys. They expect to get the chance to date the girl, simply because they want to, like the girl doesn't have the choice. She may not like them that way. She's allowed to not want to date them. How do you know she hasn't thought about it and decided against it?
There are plenty of people out there who make the same relationship mistakes over and over, and fall for type again and again with no success (I still have friends in their mid 30s that do this), but until THEY want to change their dating habits, it's not going to happen. No one owes anyone else a date, "just to see" if it'll work.
You take far too much pleasure in other people's misfortune. There's no guarantee you would've had a successful relationship with them or that your life won't go to hell. No one owed you a date, no one owed you a relationship.
IMO, rejections and failed relationships are important learning tools, because they help you understand yourself better and how you can be a better you. You learn more about what is important in a relationship, what's important in a date, what you will put up with and what you won't. You learn to improve yourself, not so that you can score better, but so that you can be a better person.
One reason some girls like older guys is because the older guys can provide the girls with stuff the younger guys can't - alcohol, gifts, nights out. Those girls aren't ones you want to have a relationship with, because they're just out for what they can get. Some girls like older guys because they're might be more reliable, more stable, more ambitious, more interested in doing things. Some girls like older guys because they have daddy issues and find security with older men.
Sitting around and being negative about all the people who have rejected you only makes you bitter, and bitter isn't attractive. Everyone has their preferences, and there's nothing wrong with someone else not finding you attractive. I don't find everyone attractive, so why would I expect that everyone would want me? I don't. But I know there'll be an overlap between the people I'm interested in and the people interested in me to find someone that I'll be able to have a relationship with.
I personally am not interested in dating someone that much younger than me, because chances are, we'll be in very different life stages. I'm at the wanting to settle down into a committed relationship point, not the dating around casually, let's see what happens point like we tend to be in our early 20s. I'd like more kids, and I've only got a few years left where that's an option to me, and someone who's ten or more years younger than me might not be in that place, where they're ready to do that so soon.