Is 18 years old to young to get a PDO Thread Lift?

If Botox lasted for six months, that is quite a long time. Usually it lasts 3-5 months for most people. I do think you are too young for a thread lift at 18. In fact you are too young for Botox also. The best advice for you is to limit sun exposure since this will serve you best in the long run. At the very least you need to use a good sunscreen with UVA/UVB protection and reapply it frequently.

A combination of both the threads and some Botox would not be a bad idea to help with the cheeks. The Botox would help to "soften" the muscle around the jaw and eyes and the threads can help to give you a natural lift. I would highly recommend using a strong sun screen if you are going to be out in the sun as much as you say you like to be. 

It is common to use neurotoxins as a preventative for wrinkles and I have many patients starting in their mid-twenties. I know you love the sun, but a physical sunscreen is and should be your BFF for years and years to come.  Look for the ingredients titanium dioxide and zinc oxide.  They will take off years of sun damage for you in the future.
As for your chubby cheeks?  Embrace them if you can!  I see woman pay a LOT of money to get their cheeks restored.  PDO Threadlift is going to help lift the tissue and lessen the nasolabial folds and jowls.  Coolsculpting Cool Mini or Kybella can help define your jawline and under the chin. If you have enlarged masseter muscles due to clinching, we can absolutely offer neurotoxins to reduce the thick and enlarged muscle there to define that area.