Seriously? Do you have any idea how much research has been done in this area?
This is a very basic overview of a segment - fluency development.
To wit -
This is another simple overview, but more specific to what you were asking.
From an entire book which addresses the subject -
All of this is meant to demonstrate what the above poster was saying. There are developmental milestones for a reason. When a child cannot or does not meet them, it's not just a matter of catching up later.
Same as the few truly feral or so severely abused children as to have been removed from most all contact show - there are windows for things, for human brain development. There have been children who were not spoken to and thus not taught to speak or were not taught to read or etc. Years later, with psychologists and educational experts doing everything possible, they do not make up those deficits.
Yes, adults can learn to read. They, in general, do not gain a level of comprehension and fluency that one would need to succeed in school. Same as if you isolated a child and never spoke to him or her in any language or pseudo language until he or she was 10. That child likely won't really learn to speak fluently, because the window for that is gone. The brain sets up neuropathways most at a very specific time. Past that, you're fighting a very hard tide.
Same as if you speak to a baby and toddler in two languages consistantly, they'll be bilingual. If you start learning a second language in high school, how well does that stick? Mostly, not well. Many people who grew up in immigrant households, bilingual from birth, though they don't use their second language as an adult and though they attended English-only schools and had English-speaking friends, can still spout fluent whatever as adults. People who start studying a language as teens, even if they get good at it then, can lose it very, very quickly, if they don't use it, because the pathways that link it to language centers properly weren't set up when they should've been.
The 'some kids just take longer, it's fine, in their own time,' does a HUGE disservice to kids. It's not true, it's pablum for adults don't want to think their kid has a problem. If a kid cannot read by age 8 or 9? That's a problem. That's not 'most kids walk between 10-12 mos and he's 12 mos. and 4 days and only cruises'. That would not be a problem. Years past when a skill should have been mastered at the outside? That's a problem. That needs intervention immediately, because that kid has needed intervention for the past four or five years and not gotten it - as demonstrated in the above articles.
As for the Fraser study, I didn't look at it and thus don't know. There could be many explanations from sampling to the kind of thing that generates those homeschooled creepy spelling bee winners - they spend all their time on that. Could also be that motived, uneducated parents can do well, dunno.
The schoolteacher who changes grades isn't "brushing up" from zero. They're educated in the areas they teach, they're educated in pedagogy, they're educated in development. That's not analagous to a parent who got out of high school 20 years ago, remembers very little history, didn't do well on it to begin with, trying to 'brush up' on a subject like history or algebra that's much more than the sum of its parts. You need to really understand history to teach history - have a comprehensive overview of many different areas to know how things in one area at one time interconnect. Memorizing dates is nothing. Same as you can memorize the formula for area vs. perimeter but if you don't actually understand... it's useless, because you'd lack the ability to build on things or use them when there's no indication of what to do.
Parents should be held to the same standard. We've got ways to test and measure the performance of and monitor and retrain, correct or dismiss teachers who do not live up to standards. Why shouldn't the same apply to parents?