I have a 2 1/2 year old who is into sewing. well as into it as she can be. she is getting a toy sewing machine for Christmas. it has a finger guard on it so she wont sew her fingers just the fabric. will wrap it in fabric. I just can't wait.
I too would try and take the teens to the fabric store so that you can at least get an idea of what colors they like and what prints. one might like really modern an other might like calico's and another might like a combo of the two. teens are just so hard to figure out these days. and i was just one about 10 years ago. wow do I feel old now. hehe

I've seen lots of peace signs and smiley faces. Look at t shirts in jr size in stores, a good indicator of what kids are into.

Ask them. The greatest gift is listening. Maybe go shopping together!

I made a sweet 16 quilt for my daughter last year. I let her pick the focus fabric and I picked other fabrics to go along (her fav colors are purples/bright yellows/bright greens). I picked a pattern I thought she would like - but she picked one she liked better - so I went with what she wanted and she loves it. It turned out to be one of the nicest quilts I've made.

So, yes, try and get them involved in the process if you can!