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.
Some tests are advisable and a proper history evaluation to make sure it is not a hormonal problem. If the cause is not hormonal then hair transplant can be done for your problem. I have done several cases of hair transplant for beard and moustache and patients are very happy with the new look. Get an online consult privately with pictures or meet directly a dermatologist who is specialised in hair transplantation for a proper evaluation of the condition. Hope this was helpful.