I am only 17 years old. How Long Do Hair Transplant Results Last?

While there isn’t an ideal age for patients wanting hair transplant and we often treat men as young as 21, at 17 you might be a little young for the procedure. We surgeons can sometimes (based on your health & family history) prefer younger men to wait until they are older and in their early 30s. The reason for this is that if hair loss is at an early stage it can be difficult to predict how it will advance.
Now to answer your other question. In theory, hair transplant is meant to be a permanent solutions to the problem of baldness and thinning hair. Even in some specific cases, if it doesn't last a lifetime, it is extremely long lasting and one of the most successful treatments to regain lost hair.
The transplanted hair follows the behaviour of the donor area and will continue to grow as it did in the donor area.
No two cases are the same, so comparing yourself to another before / after picture on the internet and imagining an expected result based on a Google search is not the right approach. Ideally, you should look for a well experienced and qualified surgeon in your city and visit him / her for a consultation.
This is because, permanence of hair transplant is something that depends a lot on suitable candidacy and your surgeon setting the right expectations. Your cosmetic surgeon should be able to evaluate your suitability based on your age, genetics, health record and family history. It is then that the surgeon and patient mutually set the right expectations.

I do not do hair transplants on teenagers that are undergoing massive hair loss.  The problem is you have no idea where the good donor hair is and you can spend a lot of money and the hair will be quickly lost.