People differ

Language capacity is innate, no doubt.  It's the phases that language goes through that make it less than predictable.  The idea is that as a human proceeds from infancy to maturity, language growth does the same.  Maturity is really easy to see and chart with the physical aspect of a human's maturity.  It should stand to reason that other human developments go through the maturing process in much the same way.

Here's the thing that schools refuse to accept, and in so doing, wreak havoc with young people's mental image of themselves.  The language people use is so diversified that it is impossible to project a trajectory for it.  People's accents differ.  People's jargons differ since choice of profession dictates the jargon learned to a great degree.  People's friends differ, thus their basic stock of words vary.  People's interests and personalities differ, so the words they choose will follow the lines of these two areas.  People's motivations, intrinsically and extrinsically, differ.  Thus, the number of words and types of words vary.  It's a ragged growth journey from infancy to maturity, physically, mentally, and yes, linguistically.

So, why people are so caught up in trying to make a cookie cutter system for language development is absurd, ludicrous, and otherwise ridiculously inane.  Making a standard for what young people should be saying, writing, and reading is cruel at best and unusually inhumane and unrealistic at worst.  No one really chides a child or adolescent for exceeding or underachieving an average weight or height for his or her age group.  That's unquestionably ridiculous.

People would do well to remember that language matures as people mature.  Because people are unique, their developments are unique in every aspect of growth.  That's what makes growth ragged. Just as healthy eating is good for a body regardless of how short or tall a person is, or what body shape a person has, so a system for good language input is important.  But the outcome of that system is not manifested in a uniform, standardized manner.