> People have been drinking alcohol since time immemorial.

This is probably the reason why there is no unified age for drinking, because everyone came at it at different times from different place and have differing rationale including social, religious, cultural etc.

Social media is new and there is no cultural/religious rule for/against it. So 16 is the starting point someone decided (was it Australia or NZ?), and others are following since it's a good starting point. As time progresses, maybe it'll move up or down and different countries might take a different stand.