No. My eldest was a really hard baby. "Highly sensitive" or something. Perpetually angry. He talked early and had nonsensical arguments. He became a violent toddler. It was so, so hard and horrible.
We tried everything. We read all the books and blogs. Took advice from our wonderful and massively experienced childminder.
But eventually, around age 4, he grew out of it. He still has his moments, but they're decreasing in frequency and intensity (though we are about to hit the teens). He's a much more robust, laid back, independent kid.
DS2 was the opposite. A relaxed, happy, laid back baby. Such a relief! Lovely happy chap. Until he got to about 4, and went in the opposite direction becoming grumpy, whiney and melodramatic. Often known as Grumplestiltskin at home.
I don't know what I was like when I was that small (probably like DS1). But I was super shy as a kid. But I grew out of it, and into myself. Probably helped by my parents gently pushing me to take opportunities, and having firm boundaries and expectations.
So, yeah, they change. People change all the time. We're never finished.