Children grow in unique and individual ways. Milestones vary based on the source you look up and the developmental process is not identical for all. That's why we created this development milestones guide. It provides an average age at which most children should acquire a skill.
Keep in mind, typical development follows a predictable progression. If your child is following the milestones progression, they are just a little behind that does not necessarily mean they are delayed. Furthermore, if your child has not mastered a certain skill by the suggested age, that also does not mean they are delayed. On the other hand, If your child has some skills below their age and others way above, you do want to try and work on filling in those gaps. Likewise, if most of the items in an age range have not been achieved, parents should consider further investigation in that area.
We hope this helps you gain insight into what you're observing in your child today and to preview what you can look forward to in the months ahead.