I've also found that my heat tolerance has declined as I've gotten older. Last summer, we attended an outdoor wedding in Oklahoma. It was f'n hot, and I was in a suit, because I'm a man. I hydrated before -- nothing major, but also probably not enough, based on how I reacted. After the wedding, we went inside to the reception....and I promptly sat down and drank a lot of ice water because I was looking pale and felt clammy. Took me a good 20 minutes of cooling off inside and drinking lots of cold water before I felt back to normal.
I can work outside in the heat if I plan for it, hydrate well, and take actual breaks. But I'm not as resilient as I was 30 years ago, when I could fix fence, feeders, and blinds all day in July.
Oh....and the euros DO NOT FUCKING HYDRATE. It's something we comment on every time we visit. FFS, would you people drink some water? Water is not around or available like it is here, they don't drink it like they do here....and I get it, 7 months of the year are chilly and damp in Germany. But the other 5 months....are not. And even during those chilly months, you're often inside in artificially heated air that dries you out. DRINK SOME WATER.