When do you take over percentages?

My polo is insured by my dad, as a 2nd car, for 6 years. I am now 24 and drive the car for 6 years. Due to some problems with the insurance we wanted to cancel the insurance of the 2nd car. I wanted to take it now also, and wanted to take over the percentages of the 6 years. Now the insurance type said that it does not bring me anything, I would get in anyway at 100%. Why that now? Do I suddenly not take the percentages from my dad, or the 2nd car? Or is it only possible when I am 25? I don’t see anymore