I started slowing down 28 years ago at age 45 when I had a heart attack that went undiagnosed for 24 hours, thus some permanent heart muscle damage. I'm now 73. I recovered from that heart attack and from several subsequent ones and continued to do long tours. On each tour I would get slower. Being leader of several ACA tours I was required to ride sweep or assign another to do so. I'd always do it because it gave me an excuse to ride slow!
But the mandatory statin drugs have devastated my muscle tone and beta blockers have sapped my energy. I can only tour vicariously now through Bike Forums and ride locally with e-assist.