Could a person have secondary polycythemia at age 21?

Yes.: Polycythemia vera, a primary disorder of blood would be very rare at age 21, but not impossible. As for sercondary causes, inherited causes of high red blood cells, smoking, lung disease, congenital heart disease, and high altitute living could easily be found at age 21. "blood doping" (using a drug to increase blood count) will cause polycythemia.