Have observed this with my grandchildren. Can't remember those early years with my own kids. Right now we're babysitting a couple grandkids in the 2-year-old range. Vocabulary increase is amazing, watching them go from simple phrases at 2 to compound sentences at 2-1/2. And substituting other words when responding to questions, that totally make sense. At 2, she might say "No" to a request to hand something to you. At 2-1/2, she'll say "Get your own because I'm using this one." Amazing how they first mimic us, and then build their intelligence to create sentences. Can't do it without interaction from parents and others helping them along.
It is becoming quite common for well educated parents with good jobs to put their babies in daycare at 6 weeks. From 8-5 these babies are at the mercy of a $8 per hr worker who has 4 other infants to take care of.