I think the rule is only one bare area at a time, so shortish dress/sleeves, longish dress/bare arms. And no cleavage. I still bare my arms, and they are good for a 70 year old. Don’t like cropped trousers, I think they are very unflattering, but I do like ankle length ones in a good cut-Whistles and Toast are my go-to brands. I don’t think there are many things you can’t wear as you get older, it all depends how you style them. There’s a good book- and blog- by Ari Seth Cohen called “advanced style”, which shows (mostly American) women and a few men who obviously delight in clothes and wear them with panache.