Does each service really need to collect this data from the user directly? They could instead have the user authorise them by e.g. OAuth2 to access their age with one of the de-facto online-identity-providers. I would be surprised if they didn't implement an API for this sometime soon, cause it would place them as the source of truth and give them unique access to that bit of user data. Seems like a chance and position they wouldn't want to lose.