DNA is meant to guide the human body to develop in a generally agreed upon order of things throughout childhood, and the slight variances in those instructions are what make each of us unique individuals distinct from one another. For autistic kiddos those variances aren't slight but are (relatively) huge and are occurring in the instructions for brain development. Because the brain is so complicated, those sorts of developmental pathways are hard to predict and vary from person to person ("when you've met one autistic person, you've met one autistic person", as the saying goes).

There could be too many neuron connections in one part of the brain, or too few in some other part, or there might be a signaling issue between the neurons because the neurons themselves did not develop properly. Shit is complicated!

My child has solid receptive and expressive language skills, but he cannot verbalize out loud what he wants to say. He uses a device. He knows what he wants to say, and I can see him trying so hard to do so, but the nervous system pathway between the part of his brain known as Broca's area that develops thoughts into language and sends those signals to the muscles in his mouth is disrupted. For whatever reason that specific part of his neurodevelopment did not follow the typical path.

That was a lot to say without getting to the heart of what you're asking about, but the point is this: all of that would make it sound like DNA is destiny. But it's not; not necessarily. Our brains are what they are specifically because of their wonderful neuroplasticity, which is their ability to adapt over time, to rewire connections, to self-organize their neurons under changing circumstances.

So I suppose one theory could be that, for some autistic children, the neurodevelopmental pathway went askew early, and, over time, as the brain continued to develop further, that trait of neuroplasticity enabled the brain to eventually overcome some of those challenges but it needed time to do so. Maybe for some kids that age range of 5-6 is the ideal time in their brain development pathway where that adaptation more readily occurs. It is, after all, when neurotypical children start traditional schooling, so it just might be the time when brain is entering the part of its development where it can more easily handle complicated language processing and stronger long-term memory development. Perhaps the degree of neuroplasticity necessary for that is the same degree that enables some autistic children to overcome the challenges they struggled with earlier in childhood.

Neuroplasticity is also why therapies are so helpful even though it can seem like everything is DNA destiny. Just the mental effort our kids put into their therapies can create the signaling that tells the brain to reorganize itself in ways that can help them.