There are many aspects that decide whether you are a good hair transplant candidate; some of them are emotional issues and some medical; both must combine to make a good hair transplant candidate. A Hair Transplant is not a cure for hair loss but simply the movement of genetically strong hair from the sides and back (donor area) to the area of hair loss (recipient area); and the donor area has a finite amount of hair.
Hair loss is progressive and although family history can be a guide it does not always mean you will follow the same pattern. If a hair transplant is undergone too early and hair loss progresses you may not have the resources to treat the area
Age, hair loss pattern, must be taken into consideration.
Technical considerations to consider revolve around the quality of your donor area, the size of the donor safe zone, follicular unit density, hair characteristics. The donor area shrinks as hair loss progresses; with FUT this is less of a problem because only a relatively thin strip is removed, but FUE relies on extracting over a wider surface area to reach high numbers, so making it difficult to measure the safe zone long term.
If the hair density is low this will restrict the amount of hair that can be removed; this is less of a problem with small procedures but as hair loss progresses there will not be enough hair to treat the area, so better not to start. Hair characteristics; a low hair to FU count does not make for a good hair transplant, very fine hair will affect the quality of the result that can be achieved and miniaturised hair reduces the amount that can be removed.
An ethical Clinic will always advise caution before suggesting a hair transplant, and if they feel you are not a good candidate inform you of the reasons why they have come to this conclusion. It maybe not what you wish to hear at the time but in your short and long term best interest.