My parents have told me that when I was two and a half, I had the speaking ability of a 1-year-old, but by the time I was three, I was speaking like a six- or seven-year-old. My dad believes (and, given how I know I was later, I agree) that I went through a long "observational period" in which I was constantly acquiring language but didn't want to show what I'd learned until I was ready.