There is a common saying: "each one has its own pace". That is true, every child has its own pace, every one grows in a different context. There are scales of infant development published by many authors, easily to find in general bookshops. Even health centers can give us material on this issue.
At early ages, 6 month of delay in the achievement of a goal requires the advise of an early care specialist. Not being able to walk when they are already 18 months old, to talk at age 2 or if they have an incomprehensible speech at the age of 3 are not necessarily red flags, but a reason to consultation. Not everybody has the same pace, but we are pleased to be helped in difficult situations.