I would say from 60s - which is a pensioner's age, having said that if they want to wear it at home that's fine.
As you probably know Stricktly, 50 is the new 40. There are a lot of ladies in their 40s, 50s who still have great pins on them. Cilla Black is one good example. (from UK) She still looked gorgeous in short skirts up to her 50s..
If you've got nice legs flaunt 'em but if you haven't...... lol.
Ps. Just seen teendad's answer. Tina Turner does have a great pair of pins on her. I guess some older celebrities could wear short skirts till they are 69 - if they have lovely legs.