Average Age vs. Normal Range for Developmental Milestones

Average Age vs. Normal Range for Developmental Milestones

A concept that often is misunderstood by mothers and fathers is the difference between “average age” and “normal range” for the appearance of major developmental milestones such as the first steps, the first words, etc. And failing to understand this difference often results in a lot of needless worry.

Let’s say I observe a thousand babies and note how old they are in months when they begin crawling. I add up all those months and divide by a thousand. That gives me the average age at which babies begin crawling. That is nothing more than a meaningless statistic, and it gives me no insight when assessing an individual child’s developmental progress. What I really need to know is the normal range. That is the entire period during which a skill can appear and the child is considered on course for normal development.

For instance, the average age for crawling is about 8 months, but the normal range is from 6 to 10 months. One child starts crawling at 7 months, another child starts crawling at 9 months. Which child is likely to begin walking first?

Most people would say, “Seven months? That is a month before the average. That child is ahead! Nine months? That is a month after the average. That child is behind!” Not so. Developmentally speaking, the two children are absolutely equal, and it is just as likely that the child who started crawling at 9 months will begin walking before the child who started crawling at 7 months. Differences within the normal range mean nothing and have no predictive value.

Unfortunately, we live in a highly competitive society and it is easy for parents to get upset when their child appears to be slower than her peers. Take expressive language. The normal range for the appearance of the first words is huge – from 6 months to 24 months. But with a smug look, one parent will say to another, “Your Sally is 18 months old and just started talking?” Our Sammy has been talking since he was 8 months old!” And what the other parent hears is, “Our Sammy is a genius. Your Sally is behind.”

So to avoid unnecessary grief, mothers and fathers should use books, magazines, Romp n’ Roll instructors, and whatever other resources are available to educate themselves about normal ranges of developmental milestones. It will certainly help them to relax, to remain unfazed by inappropriate comments, and to appreciate their child’s individual and perfectly healthy rate and pattern of progress.

For your printing pleasure, here is a downloadable PDF.