I used to get that when I was riding bmx all the time, its a form of tendonitis and nothing you can really do apart from rest them. I did find that having my brake lever bite point closer to the bars helped, also having a slightly spongey brake (rather than one that is solid at the lever). Im now 29 and have been riding bikes in a silly manner (bmx/jump/trials) since I was 16, Im certainly stiffer now and recover much slower but as dwats said, once the body has acclimatised to the work then its normally ok My endurance has reduced drastically though, now I can manage maybe 4 hours of solid riding before having to stop, used to be able to do 12 hours solid at a skatepark