I am 26 and get this way! I find that if I stitch for to long I will get a cramp/swelling in the thumb area of my left hand (the hand I hold my project with) and in my right hand fingers! I find that it is worse when I have not stitched in a long time! What I do to releave the ache is I soak my hands in the hottest water I can handle, and then hold some heated rice packs. Sometimes the swelling gets so bag I can't make a fist the heat helps SO MUCH!!
I also get this way when I decide to crochet something. Since I don't crochet very often it takes a while for my hands/wrist to get use to it.
The advice that Up North gave is great, here is a website that also lists some exersised for sewing!
http://www.its-sew-karren.com/hand-exercises.html