When They're Ready
'Readiness' is, quite simply, when a child is equipped to handle some kind of transition in their life. For some transitions, of course, readiness is not a concern, because the child alone will make the transition - a good example is going from preoperational to concrete operational thinking. Evidence suggests that many preoperational 4 and 5 year olds are ready to make the shift at any moment, just waiting for the lightbulb to flash. But other transitions are initiated by parents, such as the transition from diapers to potty, and these are the ones for which readiness becomes a parental concern.
For parents of NTs, this question can often be easily answered by age. When is my child ready for solid foods? Around six months. When is he/she ready for toilet training? Two and a half to three years. When is she/he ready for school? Five or six years. When is he/she ready to be considered an adult? Sometime between 18 and 25 years old.
These ages are given by society - other parents, various 'experts', lawmakers and policy makers, etc. They're based on experience (both implicit and explicit) with many different kids, which has shown that most do fine if a transition takes place at a certain age. Sometimes, of course, they get the age wrong. (One reason psychoanalysts focused so much on toilet training was because many parents were training their kids really early.) Other times, the age actually changes due to changing requirements - as education has gotten more essential to good employment, we've scaled the age of adulthood up to make room for it. But in general, if most parents stick to the designated ages of readiness, they won't be far wrong.
With neuroatypical kids, though, these ages change. A highly gifted child may be ready to learn to read at 2 years, and when he isn't taught then, he finally teaches himself around 4 years old and ends up bored and frustrated in grade 1. A child with Down Syndrome may not have the understanding or self-control to stop using diapers until she's 5 or 6 years old - trying to train her at 3 would be like trying to train an NT 1 year old. A high-functioning autistic person may not have the organizational skills for independent living until he's 26 years old, and only then can he safely live alone.
So these parents are left with a challenge. If they stick to the standard ages of readiness, the child suffers - like the many autistic young adults who get kicked out of condemned houses because they don't know how to clean them, or the gifted schoolchildren who tune out and coast on the easy assignments in school and then fail university because they don't know how to work hard. If they try to figure out when their child is really ready, they're navigating uncharted territory, and life becomes a guessing game. Some parents get more guidance than others (such as the parents who post on DS forums asking when other DS children were toilet trained) but none of them get as much guidance as NT parents get, because we simply don't know as much about any developmental difference as we do about NTs.
I know some kids with developmental delays who haven't been pushed to their full ability because no one knew what they could actually do. I've seen others who have probably been pushed too hard, and who act out because they're tired of being asked to do things they don't understand. I've seen kids whose mixed skills trick people into assuming that their lack of readiness is actually defiance, or cause people to miss the readiness that's actually there. I've been a kid who kept getting lost because people thought that I should be able to take the bus independently at 11 years old, when I still struggle with that skill at 22. I've also been a kid who longed for something to make analyzing data easier at 15, and then at 21 discovered the existence of SPSS. (I could probably have handled entering university at 13 or 14 years old, if it hadn't been for my traumatic experiences in school.)
If most kids were like me, I know my life would have been easier. And one big reason is that everyone would know to start teaching me new skills when I was ready - not too early, not too late. I would have learnt that most of what people threw at me was stuff I could do if I worked hard at it. I couldn't have gotten away with tuning out so much of school, and I wouldn't have learnt to give up whenever a real challenge comes along.
Alas, no one disability is common enough for the wealth of knowledge that makes it all flow so easily. And even when you can find many kids with the same condition, almost invariably you'll find that they're more diverse than NT kids are. (Down Syndrome individuals, for example, seem to range all the way from borderline IQ to severe cognitive disability, though most are mildly-moderately cognitively disabled.)
So, what's a parent to do? Well, get really good at the guessing game, of course. Learn how to spot readiness and lack of readiness from the kid's behavior instead of their age. And get good at searching out whatever knowledge is out there, so they can use it to guide them.