Err, it's exactly the same - the shared vehicle would have to be limited to the licence of the most restricted driver.
If I want to share a bike with my son (I do), it has to be under 125cc. If mum and dad want to share the car with the 17 year old, then mum and dad need to choose a car that is suitable for a 17 year old.
Mum and dad might just discover that driving a VW UP! isn't actually that bad :-)