They can already target children through existing means. This doesn't change anything for bad actors.

Pedophiles aren't being blocked from finding kids on Roblox because they don't have an OS confirmation that the person checked a box saying they're roughly a child, and legitimate companies can already do this through the numerous existing ways of determining user demographics.

Targeted advertising is not a problem that is solved by OS level security, it can only be solved through legislation targeting the actual harmful practice. A user spending any amount of time online will always leak enough data to figure out roughly who they are, especially with AI tooling that can do pattern recognition orders of magnitude more effectively than anything that came before.

OS level account setting is by far the simplest and least intrusive method of having some kind of age verification. Parents setup their kids account then the OS transmits the signal to the server. No IDs, no government involvement, no databases.