Hi abritt,
What kind of arthritis is your doctor talking about? Osteoarthritis (wear and tear) is different from rheumatoid arthritis (autoimmune disorder). RA can strike at any age. I've had it since age five. Are you having accompanying symptoms of low-grade fever, fatigue, skin rashes, redness and heat in more than one joint? Or is it only pain and stiffness when you overwork the shoulders (like if you're lifting weights or doing something else that involves shoulders in your work).
I can't say I did, but rheumatoid arthritis can strike at a young age. You will want to get endocrine disorders ruled out as a cause of your joint symptoms.