See your doctor. Don't assume it's arthritis, though it very well may be, make sure there are not other problems going on.
I am 26 and in the past few years have been developing arthritis (doctor diagnosed). I have it in my hips and occasionally in my hands. It stinks, but I've learned to deal with it. I take OTC meds when it gets too bad. I find that for me there's a fine line to walk- being too active causes pain, not being active enough causes bad pain & stiffness. The doc said that although it's not as common, arthritis can strike at any age
Good luck & let us know what you find out.