I have sleep apnea OCD anxiety and ADHD at age 18. Is it really rare for someone my age to have all those problems?

Rare enough. It really doesn''t matter much whether this is a 1 in a 100 or 1 in a 1000 question. Having all this in combination at 18 is a real problem for you and you should accept both the help of sleep specialist and a psychiatrist.
Depends. There are a lot of factors which govern this you should get yoursekf evaluated by an ENT and see a sleep doc.
Not really. I would not say it is rare. But it is several problems for an 18yo to have. Make sure you take care of yourself, eat healthy, get to and maintain a healthy weight, see your doctor regularly, and you should stay well. (some times with weight loss, sleep apnea may improve). Best wishes.