Ideally, I'd want them to wait till they are in the double digits (10 years and above) but obviously that's not happening. I've a cousin who's 6 and she has facebook, instagram, twitter and snapchat. I suppose all we can do as adults is to guide the kids and educate them on both the benefits and dangers of the Internet, social media etc.
my elder one started using the internet when she turned 8, as part of her research for school projects. however, i make sure that she does it on a monitored basis. for now, she watches a lot of DIY as well as educational videos online, but one or the other parent is always present to keep an eye on the content.
these days kids are encouraged to use the internet at a very young age, even for school projects. my kids were exposed to the same around the age of 6 and it was quite good actually. the only thing to remember is that as parents and elders, we have to make sure they are always supervised while using the same
if it were up to me, I'd only start letting them use the Internet when they reach their teenage years or later, but let's be real, in the digital age that's not remotely possible. The key is to educate them on how to use the internet and the many scary things they can find on it.
In our time now, it's impossible for them not to be exposed to the internet and social media at an early age. We even use it for our homeschool. Guidance from us and imparting discipline and wisdom is what we can equip them with as parents.