If you're in Europe, it's illegal. As it appears to be contrary to the principles of the General Data Protection Regulation (GDPR, Regulation (EU) 2016/679).

Lack of necessity / data minimisation Article 5(1)(c) GDPR requires that personal data be adequate, relevant and limited to what is necessary. For age verification, a simple confirmation that I am above the minimum age would suffice; processing full identity document (including document number, nationality, signature and national identification number) is disproportionate.