A tooth is loose. im 20. is it still possible that it is a deciduous tooth ?

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.

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.

Possibly: Over-retained baby teeth, especially if there is no underlying permanent teeth, or if there is a malocclusion, is not uncommon. If it's a loose permanent tooth there may be some underlying pathology. Please see your Dentist to determine cause/cure.