I was raised similarly to your cousins. We had fairly strictly limited times when we could watch TV, or especially, play computer games. As I got older, and started having more freedom, I would frequently use that freedom to play excessive amounts of games for an unhealthy amount of time, to the point where other important things like schoolwork sometimes suffered. The restrictions made it very difficult for me to develop self-control where tv and video games are concerned.

There are some caveats there. The restrictions on screens as a child meant that I was forced to find other ways to entertain myself, both via using my imagination and via spending time in the outdoors, which both formed some excellent childhood memories for myself and laid a foundation for some of my favorite characteristics about myself as an adult.

At any rate, I'm fortunate that I was/am able to be functional despite having the above-mentioned self-control issues; I now have a graduate degree and a great career. I'm now in my 30s and have parenthood coming (looming!) in a few months. We have not yet fully decided what our screen time rules will be.