I am 20 years old, can I have a hair transplant or should I wait?

You are correct to wait until you are older to be sure where your balding pattern will show up so you will know what you are in store for. Your family history might indicate where your balding may be going. Look at your brothers, uncles, father and grandparents on both sides of the family to see what they hair loss pattern is/was, Usually by the age of 25, you will know and you can then build a Master Plan with your doctor.