Middle-Aged?

I went to the doctor yesterday,
And said, "I'm in a poorly way!
My muscles and joints ache and ache,
I sleep all day; at night I'm awake."

"I think my hair is growing thin,
After a walk, I ice my shins.
My lower back has gotten sore,
I need to rest more and more."

My doctor calmly looked at me,
And said with all sincerity,
"My diagnosis might leave you stunned--
It's all because you're FORTY-ONE!"