I lived in west Texas for a time, and found myself riding a club TT at 115F. I was acclimated and managed. Fast forward after a few years of living in the PNW and we had that heat wave a few years back, I thought I'd be fine riding the 5 miles home at 115F, due to Texas experience. Nope nope nope. I made it halfway and fortunately there was a Target to stumble into and cool down.

Acclimation is real, and heat tolerance does fall off with age.