After puberty which is around the age of 13-14, under the influence of testosterone in the body, beard gradually started developing . For the growth to become thick it takes a few years maybe by the end of a adolescence . The density and the pattern of beard has a genetic component and it would vary from person to person. If you do not develop a proper beard by the age of 20 then you might need further evaluation. Now in your case you have already done the blood test and still if the beard pattern is sparse then that is how your beard is genetically designed to grow. You can increase the density by doing my hair transplantation for your beard. I have done it for multiple patience and they are happy with the result. If you wish you can come in for a consultation. Hope this is helpful.