(For those who read and don't know, this means very roughly FIDE 1950.)
GM by age 21, probably not. I know, people like Carlsen did it even faster, but they are not humans. With all due repect, the odds are against you being the genius of the century. I would say reaching GM someday is possible, and that's already much more than I would ever be able to do.
However, how dull and boring I might sound, I would not advise to try seriously becoming a GM. Why ? Because trying seriously means sacrificing all professional life and studies to chess, and even by doing so you are not sure to reach the GM goal. Professional chess is incredibly competitive.
Yet, congratulations for that rating at your age !
I'm 14 years old and I'm rated 1944 USCF. I was wondering if getting GM by age 21 is possible. I just thought some feedback would be interesting! Thanks