I think I have hallux rigidus at 18. Is it possible?

Rare but could. Happen if you injury to the joint causing sevsr arthitis. Thanks.
Possible/unusual. Hallux rigidus is severe degenerative changes to the big toe joint. This is uncommon in someone your age. You can have hallux limitus (limitation of big toe joint motion) that can be structural, but more likely in your case, functional. Meaning the motion to the joint is limited due to the way your foot works. Good, stiff shoes and extra support can be helpful. See your podiatrist. Dr l.