Mid 30s. Gauging by how my joints feel and how quickly I recover from physical exercise and injuries.
All of which to say: there is absolutely no way you are possibly controlling for all variables here. Things off the top of my head that could help with exercise recovery, all of which are more plausible than some poorly tested supplement:
- Changes in exercise type - Changes in exercise duration - Changes in exercise intensity - Changes in exercise form - Weight loss - Sleeping more for better recovery