A tooth is loose. I'm 20. Is it still possible that it is a deciduous tooth?

Yes. Some people don't have all their permanent teeth. I would see a dentist, because either way you'll probably need some treatment.
Loose Tooth? By the time that you are 20, a dentist ought to have informed you if you still have a deciduous tooth. Other causes for loose teeth are a low grade dental abscess, or some sort of localized periodontal disease.
Yes. Some people do not lose all of their deciduous (baby) teeth and retain them as an adult if there is not a permanent tooth developing to take its place. A dental exam and x-ray can confirm if the loose tooth is a deciduous or permanent tooth.