The reality of the impact of a reading age of 10 would also differ depending on the cause.
If it's someone with a specific learning difficulty such as dyslexia, they may well have a spiky profile with other strengths, and develop workarounds that get past the hinderence.
Similarly a person working in an additional language will often also benefit from generally stronger cognition compared to reading age. They are likely to have more scope to develop their fluency with increased experience and immersion. (Between beginners Spanish, A-level French and general knowledge, I can work out the general gist of a written Spanish news article. I would lack nuance and detail, but could access the main points)
A person of below-average intelligence will struggle most with limited access to develop, limited coping strategies and often lack of support network (e.g. asking someone with stronger skills to proof read)
It's easy to get stuck in a spiral where improving literacy is difficult. Dyslexic DS 11 finds reading draining, so it's not pleasurable so that reduces natural exposure.
We started tutoring 18m ago, not so much to get him through SATs for their own sake, but because the curriculum expects him to have that range of skills (which then feeds into other subjects like MFL). Initially his reading age was a year behind at 9 (which is quite frankly terrifying to think of ploughing through adulthood at that level) and by the time he did SATs 9m later, he was closing the gap and reached RA10 at 10yo. He scraped the expected standard in SPAG and comfortably met it in reading. We've kept the tutoring going to keep boosting his skills, and now in KS3, he's focusing on comprehension and writing. He's verbally very articulate, but can't transfer the way he speaks into flowing written form easily. He has lots of little blind spots. While English doesn't have obvious specific word order rules like German, we do have conventions e.g. "A small, brown dog" vs "a brown, small dog" and breaking those leads to clunky expression even if it's not obvious why. It's the subtlties of comprehension that he struggles to pick up on, and needs to continue developing at a pace greater than school can offer to meet his general potential.
The focus of the education system is not helpful. The obsession with reading records until the end of y6 sucks the joy out of reading. I gave up filling them in. It wasn't that we weren't reading. We used audio books, sharing reading. Trying to keep pleasure in it and exposing DS to text beyond his reading age that he couldn't decode on his own.
The KS1/ KS2 emphisis on petty points of grammar is another joy sucker. It's not that grammar isn't important, but it's too soon. Children need to be confident to read and develop creative ideas first before they can really grasp the grammar that binds the text together.
Many children will come out of school having read little more than Kipper, Biff and Chip in terms of reading whole books. Schools increasingly don't have the budget to provide (or time to teach) whole books. GCSE texts are often reduced to studying some scene/ chapter excerpts and filling the gaps with the film version. DSs' school has an extended tutor time to allow free reading, but the reality is that despite being bought books to match their interests and ability level, they take in stunt books, open at random, skim through some pages and repeat at a different point day after day.
My children are lucky ones. They've grown up in a reading friendly culture, we've been educated enough to recognise the nature of their difficulties, we've been able to pay for dyslexia assessment (there are no poor dyslexic children in my LA... if you don't have spare £££, tough shit 😞) we've been able to pay for aids such as prescription tinted glasses to ease eye strain and migraines, we've been able to pay for support. There are thousands and thousands of children out there struggling because their parents are not in a knowledge and finance position to invest into it, and the education system doesn't have the means to identify and target that level of support and aids.
I can't solve the way their brains process written information, but I can contribute to closing the gap so they're not behind and struggling to keep up with accessing information across the curriculum.
The strongest indicator of a child's education sucess is the education level of their mother. That means literacy very easily becomes an intergenerational issue.