The way you describe it it most sounds like there are some imbalances that may be causing problems in your joints, or there is just general instability and lack of strength in your joint structures. I have some 60 year old clients rocking some pretty good weights without a problem. There are some changes as we get older but honestly 36 is fairly young to experience these. Typically what I tell my clients is that getting older mostly means you've just had more time for things to go wrong and wear down, not that getting old has as many actual effects based on age itself. What have you been doing as a job for the last several years? What kind of activity and exercise? What causes additional joint discomfort or pain, and can you describe the discomfort/pain/fatigue you experience?
Perfection in motion. BS CPT - NPTI NASM
"We must not forget that even in the most perverted and cruel human being, as long as he is human, a small grain of love and compassion exists that will make him, one day, a Buddha." -Dalai Lama