Childhood

I have found myself wondering recently - when do I stop referring to my son as a "baby"? He's right in that join between babyhood and childhood and sometimes I wonder if by still thinking of him as a baby then I am in the wrong.
After all, he no longer just lies there. Ah, I miss those days, when I could put him down and go and do something, safe in the knowledge that he'd still be in the same place when I got back and not knee deep in trouble. He can finger feed - for the most part - and is certainly not the floppy necked, un-focussing, tiny, delicate infant I was presented with on the first day of his life. He smiles and laughs and can get around on his own two feet with only the odd mishap.
He's a toddler, but for all that he still feels like a baby. He still needs two good naps in the day and his mobility itself makes him vulnerable as he simply has no understanding at all of any dangers around him.
Perhaps that's it, rather than physical development I should think in terms of his understanding. When he becomes aware of the fact that not all the things around him are toys and some things can harm him, when he learns the rules of cause and consequence and when he can effectively communicate with me then I can stop thinking of him as a baby.
Unfortunately I have met a number of adults who have not reached all of these goals, especially the last two. Most teenagers fail them, after all.
Indeed, the dividing line between child and adult isn't hard and fast, some teenagers are mature and sensible people and some in their twenties are total idiots. Just because someone has reached their eighteenth birthday they don't suddenly become a mature, suit wearing, sensible person over night.
Perhaps I should stop thinking of him as a baby when he no longer fits in the biggest baby-gro size I can find. It's arbitrary, but then so is everything else.
One day I'll wake up and realise that he's grown up. I'm not sure if that makes me feel good or quite sad.