I don't want to sound like an "idea guy", but coming up with ideas is easy for me. I'm ancient (30) by HN standards, though.
We tend to focus too much on youth, but often good ideas just come from experience. If you actively fight the tendency to get set in your ways and complacent, you'll be better at coming up with ideas as you get older.
For me, the hard parts are:
1. It being a good idea doesn't mean it will be easy to capture value. The inventor of Tetris didn't make a dime until the late '90s. I wrote a card game that I know is very good but I have no idea how I'd make money off of it without compromising its integrity. (I don't want to create an app and "monetize" by sending users over to Zyngarbage.)
2. Selling a great idea is brutally difficult. If it's obviously good, you have to worry that people will steal it and give the work and rewards to their buddies. If it's not obviously good (and most good ideas qualify) it will be a bitch to sell.
3. Then there's the work. That's the easiest part, except for the fact that until you've had success at #1 (coming up with an easy way to capture the value so you can concentrate on making it great) and #2 (selling it, well enough that you can dedicate some time to it) you are going to struggle even to sell yourself.
How do you find problems to solve? Live. Get annoyed at shit. See things missing in the world. That's the easy part.