@ConradMcKlusky , welcome!
They probably arrive at stores a day or two old, and sometimes are there for a few weeks.
If you have a kitchen scale you might be able to get a weight on them and some of our more numeric-minded experienced duck folk can help you figure out their age. A photo would also help.
90F is only for the first week. The temperature needs to be lowered by 5 degrees F per week. So at three weeks, 90 minus 15 is 75 degrees. And that's a ballpark. But it worked very well for us and the Runners.
So the exact number is not critical, per se, but if it calculates out to 75F, then no lower than 70, in my opinion, and 80 might be too warm. Having room in the brooder for them to move away from the heat is good. With our eleven, I found that some of them want it warmer, some want it cooler.
Giving them time in water up to the tops of their legs, and the same temperature as the brooder was helpful for us. I read an academic paper that reported that giving ducklings time in the water (supervised at all times, and not for too long) initiates preening behavior, that helps them get their oil glands going sooner than if they don't have time in the water.