This is kind of a strange question for the Modern Kilt Wear sub-forum... the answer might be "there are no rules, do what you like." I see a lot of people who wear their kilt with scrunched down socks and boots/shoes have the kilt mid-knee or lower.

But you seem to be asking about tradition? Traditionally, the kilt is worn anywhere from an inch above the knee to the middle of the knee cap. A general guide is to make sure that your knees are visible! This applies regardless of whether the tartan is hunting, modern, weathered, etc. and is independent of the age of the wearer.