Joined by a common interest, ability or talent young community members can start a sustainable business with what they have near.
They could recycle old clothes, create park swings from tires... Teenagers are adventurously creative.
I think you're right. This two populations have a lot of potential and skills.
And I reckon is a good way for young citizens to acknowledge that one day we'll be 65 too.
I also found Clara's concept enriching http://www.openideo.com/open/vibrant-cities/concepting/granny-market-/ along with Meena's amazing website suggestion http://theamazings.org/ where young bring the benefits of technology to retired people who normally aren't familiar with online tools.