One way is the California approach which requires that device operating systems offer parental controls that parents can set up when creating accounts for their children that will provide an age bracket to apps when the children are using the device. The California laws requires that apps that need to restrict use by children to ask for that age bracket.

Note that the California approach does not actually do any age verification. The parental controls accept whatever the parent says is the the child's age bracket.

Another way is to put actual ID documents on the device, cryptographically tied to the device, and to implement a protocol by which software on the device can prove to a remote site that the device contains such ID documents and that those document show an age that is in the age range that is allowed to use the site but without disclosing to the site any another information from the documents. Google, Apple, and the EU are all using and/or working on this type of approach.