What are the chances of getting throat or oral cancer at 21?

Low. It is rare for a 21 year old smoker to have oral cancer but the risk rises as you get older and continue to smoke or use chewing tobacco! another cause of oral cancer in young people is human papilloma virus transmitted thru oral sex. I have treated non smoking patients in their early 30s for tongue and throat cancers.
Throat Cancer. That is a hard question to answer. I have never heard of that since throat or oral cancer is usually related to chronic smoking or chewing tobacco, and at your age, it depends on how long you have been smoking. Stop smoking. There is nothing good that can come of it. It is obvious that you are concerned enough about yourself that you are worried. Get the help necessary to help yourself quit!