It would be largely useless. Most children's development is extremely varied. Books like to use average values but they never talk about how wide the standard d

It would be largely useless. Most children's development is extremely varied. Books like to use average values but they never talk about how wide the standard deviation. It's so wide that an average is actually a useless value. Each child is completely different, so knowing the milestones is basically useless. My son was slow to smile, and very slow to walk. But he was evaluated with a 160 IQ at age 4. I have a friend who didn't talk until he was 5 years old. His parents were afraid he was developmentally delayed. In fact, he went to Berkeley for undergrad and Stanford for his PhD and you never would have guessed he didn't talk until he was 5. So development speed is extremely wide standard deviation to the point where data from the first several years is fun if you want to brag but useless for the purpose of generating any useful data on whole, except for the severely delayed.