This is a terrible question as it reflects a doctor who is not ethical and a patient who is naive. A 21 year old might have early crown hair loss and frontal hairline loss along the hairline (Called a Norwood Class 3 Vertex pattern) which is not uncommon in a young man under 23 years old. Most of these men can stop their hair loss on the drug finasteride, but of course the doctor can’t make money that way, so this poor fellow had a hair transplant and will be destined to continue having hair transplants until is final balding pattern is evident (about 26 years old). If he does not have the donor density for this, he will look freaky as he runs out of hair chasing possibly the impossible. Ethical doctors are in the business so if you are under 25 and have hair loss, see on of the ethical doctor who does not rush you into surgery to pay for his new car.