Optimal age starts at 13. Bone matures with age.
At 21, treatment is definitely possible many people get it done at this age.
The only concern is duration of treatment. It may be some bit longer than any 13 year old patient and obviously the social issue of visibility of metal braces. Usually people choose ceramic braces at this age.
Next Steps
Must consult with a dentist. Get treatment done for better looks and function
Health Tips
do keep good oral hygiene with malaligned teeth as cavities grow faster in such teeth