At Leo's last well check, his doctor was amazed at the things he was doing and considered him to be advanced. When we're out, others are often amazed at him and are surprised to find he's younger than they expected. To us, though, he seems normal. After all, Charlotte did the same things around the same time, as did Kieran (with some variation, of course).
Because we don't follow societal norms, I can't help wondering if that at least partially explains the difference. We don't use playmates for longer than a few minutes, and there's almost always someone interacting with Leo when we do. He's often in my arms or being worn, both of which work his muscles and provide stimulation since he can see what I'm doing. We talk to, not at, our children. While we're far from being alone in these things, those of us who do these are still a minority subset of society. Many still put babies down instead of carrying or wearing them, and don't speak to them really.
I suppose it's similar to the difference between those who do elimination communication (ec) and those who don't. Those who do ec have children who are (at least mostly) diaper free. The child gives signals and the parent responds; the parent also cues the child. While it may seem advanced for Western society for a child to be diaper free from such a young age, those who do ec will tell you it isn't that the child is advanced, it's that this practice allows a child to do what is in fact normal. (One of these days I really am going to get with it and do ec).
I'm also reminded of how the differences between breastfed and formula fed babies are viewed. Those who are breastfed are spoken of as having certain benefits, when in reality this is the norm.
So, are my babies advanced? I'm not so sure.
(This is not intended to be a judgement on anyone's parenting, just me musing about societal expectations)