Google and Apple certainly don't benefit from this - they can serve more ads, track more data, and assume you're authorized to spend a gazillion dollars in a game if they don't know you're a child.

One example of this was last year when high-profile apps like Candy Crush Saga and Clash of Clans were found to have privacy policies on their websites restricting users to 13+ so they could track and advertise more while their Android and iOS apps were designated for all ages so they could get more downloads.