I have used all these methods way before I was a teen myself. Infact my first earning was of Rrpees 27 that I earned from selling badges when I was in grade 7 and of 12 years old.
Teen age is smart and eager. The following might help them:
1. Earn through your hobby.... Extend your hobby to earn. If you are good at a sport, offer couching the basics.
2. Offer baby sitting
3. Have a garage sale of the toys you are not using now.
4. Make your parents help you sort out the things un used for a longer time period and sale them out.
5. Use sites like www.olx.com of www. flickr.com to sell your products.
6. Make a recycle project out of house hold junk and sale it.