From what has been discussed above, it’s pretty clear that the exact time and age to move out of cadets isn’t super clear.
However, food for thought: Recently, I was at an event in Los Angeles, and one of the drivers under our tent was a pretty talented/quick racer. Usually, he was up towards the front of the pack. However, on this day, he was nearly bicycling the kart in every corner. I mean, it was bad. So we kept taking grip out, kept taking grip out, and eventually it was somewhat better, in combination with altering his driving. Worth noting that the kid was on the larger side for a cadet, and thus we concluded in addition to over-driving, his height and weight were causing the cadet kart to flex beyond the point where it could really roll through corners with proper performance.
So, the point I make is this: If you find yourself at some point encountering the kart being massively over-stuck, or struggle massively to cut enough weight off the kart/driver to keep near the min. weight requirements, that may be a sign that it’s time to move on from cadet.