The ownership of the car isn't so important - they ask that as a separate question - the important point to the insurer is who is the main driver.

They then assess the risk of having a 40-something year old driving the car most of the time and a 20-something only having occasional use.

If the young driver ends up having almost exclusive use of the car, that increases the risk simply because he is on the road more often. And if they can prove you lied about who the main driver is, that is all the excuse they will need to refuse to pay out in the event of a claim.