I think it gets very tricky after 50, fashion that is, well let's be honest most things do :) Before that finding, trying and buying clothes felt quite instinctive. Now nothing too short, frilly, girly, overtly sexy and definitely NO LEATHER, for me anyway.
I think vintage/repro vintage maybe the answer, the new Prada Le Voyageur Summer collection would look great on just about any age. Very expensive obvs, but as a reference very inspiring.
The collection is big on natural fibres, dresses are made from stretch cotton poplin, and the prints themselves feature elements of stylish European cities, London, Venice, Milan and Paris. The bags are crocheted raffia, and the shoes also look to be mainly fabric too.
I'm pretty sure similar dress patterns could be found and some bright and breezy cotton fabrics would be most peachy, markets are good for these or of course ebay.
I've already got some Summer wedges that I purchased last Autumn in the 'Office' end of season sale, they're not as lush as the Prada ones but I really like them. Buying out of season can pay huge dividends, but there is an element of luck finding exactly the right thing in the right size.
They're in a lovely soft turquoise nubuck, are wonderfully comfortable (another concern after a certain age), and only set me back £15.50 including postage.
top image of Lucille and Peg found at unefemme.net