Usually when it gets above 24/25 C we start to really put on the air conditioning. And below 20 C we put on the heat. A/C is generally important for not only cooling, but reducing humidity so your body's sweat actually does something useful.
I've lived in houses around here that didn't have A/C and it's pretty dreadful in the summer. You basically don't do anything during the day, and it's too hot to really sleep comfortably. Even fairly poor people usually end up with at least one room with A/C.
This isn't true for all of the U.S., the Pacific Northwest (e.g. Seattle) and Northern California tends to not have air conditioners because it doesn't usually get hot enough.
Quite a bit of the U.S. is at the same latitude as North Africa or the Middle East, the northern-most bits get maybe to Belgium? So even with the temperature gradients that ocean currents and global air movement affords, we get total sun that's more like areas of the world people associate with being "hot".
Having been to Europe numerous times, I notice most public areas, cars, buses, trains and offices seem to have A/C. So I'm sure you spend more time in it than you may realize.
I'm pretty sure we don't put our heating on here in Edinburgh until October when nights are probably getting to be well below 10C.
Mind you for me 10C is "comfortable", 15C is "warm", 20C is "hot" and 30C means I'm on holiday somewhere hot where I can go capsizing to stay cool.
However, I went to university and stayed in centrally heated halls of residence, which I didn't find warm. However, when I returned home for Xmas it felt like I had been shipped to Siberia - I was dying of cold! I remember being pinned to the bed by a pile of blankets as I desperately tried to stay warm at night.
Some corners of Athens are disgustingly unbearable. Tall buildings, narrow streets filled with parked cars. The entire thing is a dust, smoke and heat trap. Temperatures feel 10 degrees warmer in those corners. Not to mention it smells horrible.
That's the reason some houses have not AC in the south of Europe but all the offices have.
Don't underestimate the thermal power needed for just one additional degree in a typical American house with poorly insulated windows, walls and doors.
We don't put the AC, we open the balcony door and the air current is enough.
In winter, we reach at most 5C, we don't put also the heating. Some neighbors have it very high and in our apartment the temperature is 20-21C.
We tend to use AC if the outdoor temp is above 30C, so most days November to April.
It gets to above 45C here, which is pretty horrible without AC.