Should a young runner take glucosamine chondroitin?

You can, but...: Glucosamine & chondroitin might help w osteoarthritis pain; the american academy of orthopedic surgeons recommend against prescription of these for knee oa as a randomized controlled trial showed no benefit; there is no evidence at present to show they can prevent future joint problems; train intelligently to avoid injuries; you can try g/c, but note they will be of theoretical benefit. Good luck!