graphics,
Do you suspect that you could have an MD such as limb-girdle? Is there a family history? I am a 48 year-old male with LGMD. My dad had it also. I lifted light weights in my 20s. But I never gained muscle mass like my friends would.
At this point it is probably ok to lift weights. Once the symptoms of muscular dystrophy strike a person should slow down on the weight training because you cannot replace damaged muscle. As you know, in a healthy person muscle breaks down during weight training then is replaced by even more tissue. Tissue is not replced in MD patients.
In my case and in many adult cases, the disease did not really kick in until 30 or 40 years old. I feel that it's not a bad idea to build a bit of muscle mass now, while you can. It can only help in the future if you feel you may get it.
Let's hope you never do. Best of luck,
Mark