We have 3 kids. They’ve all developed differently. I think milestones should be looked at more in the general sense. Not actual hard mile markers to make sure you hit.
Our 8-year-old reads better than most middle schoolers, but he can’t tie his shoes. When playing Mario kart he regularly finishes in the bottom 5 out of 12. His 6-year-old brother regularly finishes in the top 3 beating him handily.
One developed mentally and socially quicker than his peers. The other developed coordination quicker than his peers. Both are “ahead” in the somethings and “behind" in others.