I can attest that I'm definitely more affected by the heat now that I was when I was younger. I did a bike tour a few years ago when the temps were hitting the 90s (this is in Michigan, where that's relatively rare -- or used to be), but out on the roads in the sun I wouldn't want to guess what the temperature radiating up from the asphalt was. I thought I was doing OK until I got to the day's finish and found myself lightheaded and dizzy. Even a cold shower didn't totally relieve that feeling and the thought of spending another night in a tent when the lows were in the mid-80s was too much for me. I called my wife and she drove over and picked me up (to her immense relief also!).
It's been hot here again the last few days -- similar to that other year -- and I'm already a bit anxious for a ride I'm signed up to do in early August. Fortunately, this one is a couple hundred miles north of here and at the end of the day I'll be in a nice, cool hotel room, so maybe not too bad. I'm also not too proud at my age to use the SAG van to skip a segment if I'm feeling the heat.