Really not possible to give an age clip I'm afraid - it depends entirely on the woman's legs... and actually the rest of her as well now I think about it! The important thing is that it looks appropriate to the woman's overall style and appearance - there's something uniquely terrible for example about seeing a slim woman with great legs and amazing long hair from behind, who turns round to have the face of a 58 year old scrawny old witch. A woman with good skin, a good figure and the right attitude can look amazing in a mini skirt well into her 40s... many teenage girls with legs that are either too big fat and wobbly or too stick thin look appalling. What looks good is all about the individual, not the number.
Whenever they want to. You're a senior citizen? Wear miniskirts. You're wheelchair-bound? Wear miniskirts. Hell, if you're a dude and want to wear miniskirts, go for it. Unless you live in Saudi Arabia, what you wear is up to YOU.