Thank you for posting your query here. Welcome to the 50s club :)
Well, its not age related. Getting weak or getting illness is not age specific. I've seen males and females in their 70s being more active and energetic compare to there grand children. Having said that, I suggest you to consult your family physician. There are many factors which needs to be ruled out. Check if it is due to changes in climate, diet
, sleeping pattern, any deficiency (D3/B12) or any life issues. Start treatment immediately once you find the root cause of your problem. For further assistance, you can approach me or any other mental health professional of your choice. Take care.