People have been drinking alcohol since time immemorial. Our laws need to overcome those longstanding cultural standards that vary greatly across the globe and therefore laws will be different too.
It varies by country, but I would guess most political leaders didn't grow up in the era of social media, so there isn't some ingrained belief that kids actually need this stuff. And with growing globalization, it makes perfect sense that many new laws would be similar because they are both motivated by the same factors and can be used as examples for each other.
We didn't need social media before it existed. If no one's on it, that sounds like the ideal situation for young kids.
> 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.