[hnick]

If you want a better look at the basics I've found LiveOverflow on YouTube to be entertaining and fairly accessible for myself.

Fundamentally though, you will probably need to build up some low level knowledge of whatever you are targeting (whether it's an app platform, TCP packets, C/ASM code, etc).

If you have a web background there are lots of sites that have fun challenges you can start with. You could try Google's XSS game: https://xss-game.appspot.com/

It's a field that really interests me since I always liked puzzles and games and am happiest at work debugging tricky issues - but I'm not sure how easy it would be to break in as a 'junior' at age 35+.