In my past five trips to Rome over a six year period, no head covering was required.

In terms of what people wear to church in FL, it is truly awful

Well - in terms of religion I'm all for content in place of superficiality. But I do think that there is respectful attire for all sorts of occasions. (Otherwise we would see people at funerals in swimsuits.)

And in a foreign land it's only polite to follow the local definition of respectful.

But - head coverings for women in Catholic churches have not been required for about 25 years - anyplace in the world that I'm aware of. (And men - yes, do take off your baseball caps. God - or the other visitors - don't care how much hair you have left.)

We visited and I and my husband wore decent length walking shorts and I had on a sleeveless top (not spaghetti strap). Just dress decently . And honestly isn't that how you should always dress in public? Not sure why someone thinks we should "dress like the Europeans who don't wear shorts unless they are going to the beach"...when you're a tourist you are out walking and climbing all day and I'm not so certain that it would be very comfortable doing so in long pants in summer and not very advisable to do it in a skirt. Shorts that are modest and decent should never be a problem. Just my opinion though.

I am quite surprised if they let you in St. Peters in a sleeveless top and long shorts. I have seen folks turned away similarly attired.

I personally would never wear shorts in Europe when in a city. Linen pants and longish skirts are perfect for me in summer and quite comfortable.

I prefer to dress for the location and shorts for me are not something I consider appropriate. It does matter to me to wear what is culturally acceptable no matter where I am. When in Rome...

Are the rules about no shorts in churches true for young men also? I have 2 boys - 14 and 21 - who will be with us in Italy in June and I wasn't planning on having them pack long pants.

The rule at St. Peter's is no shorts for men or women. Some members of the "clothing police" stationed outside the Basilica are stricter than others. Some allow knee length shorts on both men and women (but more likely men). But the majority of guards will turn away anyone over the age of 12 wearing shorts. So yes, your grandsons need to wear long pants to St. Peters, or any church they visit in Italy. You will undoubtedly see lots of tourists in shorts and sleeveless shirts in other, smaller churches. Just because these churches don't have "clothing police" to turn you away, does not make it right for anyone to enter that church improperly attired. Some elderly parishoners may chastise the shorts wearing tourist.
Head coverings were done away with forty years ago with Vatican II.