Err... "Most" is doing some heavy lifting here. 51% of parents do use parental controls on their kid's tablets, and 47% on smartphones.

And there's no breakdown by age. Kids don't magically become able to handle the uncensored internet the day they turn 18.

Did it ever occur to you that parents who don't use restrictions maybe have kids that are almost 18? Or parents of kids who have shown themselves to be responsible? Or that the parents use other methods to restrict use (like only allowing supervised use with the parent for very young children)?