We've been told that we will look like American tourists (we are) and
stick out like a sore thumb ...
I have decided that there are two perspectives on this. One is all the other tourists will be wearing shorts and that has a lot of truth, so why not? If there are enough of "us" then "them" really doesn’t matter. In Prague there will be more than enough tourists to alter any cultural norms. Then there is the point of view that reflects on what the local population thinks of a lot of slobs wandering their streets. Not that all shorts are slobbish, but I saw a bunch of that in Rome a few weeks ago. Choose your perspective, it’s up to you.
I live in Budapest. The locals under 30 wear shorts, and a lot. Especially the men. Women seem to enjoy style a bit more than the guys, so yes, but not as much. But the locals don’t wear the ratty frayed cargo shorts that Canadians and Americans (and surprisingly a lot of Germans) seem to love. We have a tremendous number of Spanish tourists this year (and last), they dress impressively nice. The locals and the European tourists (except maybe for the Germans) over 30 not so much for shorts.
If you are a 45-year-old man and you are wearing shorts ... you are a tourist, but probably not a tourist from Europe (except maybe Germany). That’s fine, no one much cares if you are out doing the tourist walk. Just if you decide to go out for a nice meal or the theater or any place adult, you might want to change pants first. Or not. Up to your perspective.