My best years were between 40 and 47: my body was still mostly problem free, I was already financially secure, and I had enough life experience to know stupid when I saw it.
Then at 47, the progressives came onto the scene. Then shortly after, the small aches in the morning for no reason. Then I started to get a gloved finger up my ass on a regular basis to check my prostate...
That's also when I realized aging comes in bursts, and my first big burst was around that time - just before 50 - and it fucking sucked. That's the first time I felt limited in some of the things I did before, and that's the first time I felt on the decline for real.
If you're 40, enjoy it bubba, because it'll be over before you know it.