I like to try to anticipate the pattern of hair loss that you are going to have. To do this, I will perform a HAIRCHECK measurement which will give me a hair bulk analysis of your hair in different parts of your head. Usually, the value of this is limited in men under 26, but if you have obvious balding present now, the the age of 25 is not an unreasonable age. Assuming you are Asian, Asian's don't have the best donor densities (usually 20% less than Caucasians) which means that if your balding pattern is greater than a Class 5A pattern, you might not have enough hair to cover the entire pattern (worst case).
The best age to do a hair transplant is when there is a clear cut, demarcated area of baldness on the scalp with no active hair fall in which the results can be easily appreciated. Usually the hair fall pattern and the area of baldness starts becoming demarcated around the age of 23 to 24 yrs. of age after which hair transplant can be done. However in rare cases like that of traction ellophesia where the baldness pattern is clearly demarcated surgery can be done even at earlier age. But it is always better to get a proper assessment of your donor area by a hair transplant surgeon and then to plan for the most important parts of the first so that you can have sufficient number of grafts for a lifetime.
Men often begin seeing signs of hair loss in their 20s and begin looking into hair transplantation if they are uncomfortable with their appearance. That said, age isn't a primary factor; a "good age" is one where you are healthy enough to safely undergo the procedure. If you are considering hair transplant surgery, consult with a qualified physician who is experienced, uses proven techniques, and who can answer your questions. Good luck.
There is no rule on "what age to have a hair transplant". Everyone is different. In general, young men in their early 20's are not the best candidates.
There really is no specific age to get a hair transplant. This vary considerably from individual to individual based on your balding pattern. The best is to be examined so that we can determine the way you are balding and accordingly plan the procedure (hair pattern) to your needs. Another important point is determining whether or not you have good donor areas.
Usually you want to be in your mid-twenties to do a transplant but some people do it earlier and later. It's really case-by-case and you should get an in-person consult