Look to the older generations of your family for a history of arthritis. This can be a good indicator of your chances later in life. Over a long musical life of repetitive movements expect some wear and tear in muscles and joints. Probably worse, especially for knees, are impact activities like jogging on hard surfaces or old injuries (breakages, sprains, etc) coming back to visit you later in life.
Tendonitis and muscular aches and pains are another issue, and can arise at any age.
Prevention is not always easy and cure can need careful management, especially if you are having to continue playing.
Yes, there are a series of stretches and exercises, not a lot of them, that are worth doing on a regular basis, and I am happy to fax some of them to you if you send me your fax number via a PM.
Professor Earl Owen, a prominent hand specialist who takes a keen interest in musicians' health here in Australia, advocates buying a good pair of jogging shoes and striding out walking for an hour a day, to circulate blood through muscles properly before starting to play. Also in breaks, get out and walk. Another great activity is swimming, with no impact and plenty of stretching exercise.
Understanding what constitutes good posture and good muscle use should be part of every student's tuition in an ideal world. Practising in front of a mirror is a good idea so long as it is not an obsession (obviously it helps constantly monitor your technique too). Especially, take care of your back if you are tall.
We talk about warming up before getting down to some hard work. There was a recent site on this Orchestral Technique forum (Your Practice Routine, posted by "thedbass", 11-21-2008) asking for peoples' warm up routines. From memory few if any of the replies, including mine, mentioned stretches or exercises as part of their routines. A better description might have been "tuning in" mentally and physically. Earl Owen's idea is to have blood generally coursing through your whole body before "warming up".
While I was trying to make the jump from Industrial Chemist to professional Orchestral Bass Player I was doing 30 hours practice a week on top of my 40 hours a week day job plus travel. If I knew then (nearly 45 years ago) what I do now I could have halved the practice time for twice the results.
Now after many years of teaching I have distilled the process down to the acronym HEE, short for Hands Eyes and Ears. The eyes are trained to read music fluently, the hands are trained to produce musical sounds and the ears are trained to pre-hear, hear and evaluate the results. If your practice is very clear minded and focussed you can achieve a lot technically in shorter practice periods. There is much more to being a well-rounded musician, as people above have been saying.
Good luck with your future.
DP