Over the years, a lot of data have been collected on when babies and children master certain things (like walking) or start certain things (like eating solid food). This data then show the average ages for mastering these things and can then be used to provide a comparison for babies and children. So far, so good.
Sometimes, though, these averages are then taken as being some magic numbers for when these things should be done. When this happens, the fact that these numbers are averages, meaning that there were babies on either side of that number, is often ignored. Instead of just giving a general comparison benchmark, it becomes a ruler by which a child must be measured and declared advanced, average, or behind. To an extent this can be done well, such as if a child isn’t just an outlier, but off the chart in one direction or another, but if it doesn’t allow for variation, it is flawed.
At the infant stage this is perhaps most evident in the benchmarks for starting solids. With the exception of BLW literature, most things I’ve seen have had a specific age at which solids should be introduced without considering developmental readiness. The guidelines vary, but the idea of some specific age being ideal is usually present. This becomes obvious at well checks, when parents are asked about solids but aren’t asked about the child showing signs of developmental readiness in most cases. I do realise time is short at such appointments, but it doesn’t take that long to quickly go over the signs of readiness instead of just saying “start solids at x months”.
I suppose my point is that such charts of averages are helpful for the purpose of comparison, but they shouldn’t be taken to mean there’s some magic number for when a child should do x, y, or z.