I agree with @westridge with regards to age. It’s my understanding that cresties can make it to their 20s. (If I am wrong someone please correct me.)
For reference, I have many elderly leopard geckos turning 18 this year, and my herp vet has indicated that it would be ok to give a tiny bit of the joint supplements you would use in mammal pets, though there is almost no evidence for or against available to reference for reptile patients. Specific product mentioned was Glycoflex. However, leos & cresties have very different dietary needs, so it’d be best to check with your herp vet, especially with regards to dosing. Doubly so if your supplement has omega 3 FA, which can be overdosed in mammals (I do not know in reptiles).
Edit: when it comes to my geckos, I never purposefully let them suffer. When they have poor quality of life, either from age or a health condition that cannot be treated, I always take them in to be euthanized. Do not take your reptiles to a dog/cat vet for euthanasia, you are likely to have issues. Take it from a dog/cat vet that it’s always better to go to an exotics vet with your exotics.