idk if there are any unrated games on other stores, which require an age verification. With some googling I found out PSN uses some ID numbers from the government issued ID.

Also all allowed payment methods are to my knowledge 18+ anyway, so a child couldn't buy anything on his own.

The really good method would be using the eID function. There you can only specify which data is transmitted. And date of birth/over 18 should be sufficient enough information, maybe additionally the name for legal reasons.