Diolch am pob helpu!
All really good advice, thank you. It makes sense now 
A couple of things seem relevant here, in terms of what I’ve seen. It’s known that early exposure is key with language development - some studies have suggested that children from homes where they regularly interact verbally with adults can have heard over a million more words (not different words, but counting all times they’ve heard a word spoken) by the time they start school than children from homes where they do not regularly interact verbally with adults. This difference then matches to a lot of standard measures of achievement, as well as to levels of confidence and breadth of usage in the mother tongue.
We’ve also seen some evidence that working memory is key to using the SSi Method - and while I’m aware of studies that show improvement in working memory on specific tasks, I’m not aware of any that show improvements on different working memory tasks - so it’s not clear how trainable working memory is.
So I would expect that someone from a verbally impoverished home who also has a below average working memory would find learning and using a new language more difficult than someone with wide early exposure to their first language who also has a strong working memory.
Those two markers aside, though, I’m not aware of any other non-psychological distinguishing factors - and I believe psychological factors (like willingness to make mistakes, willingness to keep trying, etc) are significantly more important than working memory and early exposure…
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