Probably not in the way you're thinking.
I'll add the caveat here that if Abbie were left completely to her own devices basically 24/7 - think in something like an institutionalization from the 1950s type of situation, or something like
Genie - she would actually "regress" to something like a true feral state - and I mean that in the clinical sense, not in the sense casually used here.
If and when the Youtube grift finally dies, and if third parties like doctors, therapists, Summer, Isaiah, some of the grandparents, etc remove themselves or are removed from Abbie's life, and if they don't have her in a group home or a day program or something - if Abbie is left solely to the "care" of her parents, she would lose the skills she currently has with them leaving her to rock on a couch in the shack day and night. It's not like she would need to drink from a bottle and forget how to walk, though.
She would gradually stop asking for things (she taps and pulls and gestures and vocalizes at people to sit with her or get her food/drink or indicates she wants to go for a ride, stuff like that), the few signs she still uses correctly would be forgotten (look at the sign for 'bathroom' as an example - since this was
never consistently reinforced with her, she no longer uses it to actually mean bathroom), she'd lose any desire or ability to control her impulses...but she wouldn't be a 6 month old in an adult's body doing that. She'd still be vaguely at a toddler level - crafty enough to wreak havoc and try to entertain or comfort herself, she'd know where things like the food is kept, she'd take out her frustrations in a toddleresque targeted way.
Asa wouldn't let that happen if anyone is around to see it, though. He'd never let anyone think he was less than an ideal (in his mind) parent. Even when his YT dream draws to a close, as long as Abbie's grandparents and her doctors and even Isaiah are somewhat involved he wouldn't fully neglect Abbie in a criminal sense.
Priscilla? 100% would. If Asa dies first and she can't immediately ship Abbie off to the first facility that will take her, that poor girl is gonna be moved into the shack permanently until a spot opens up somewhere,
anywhere else.
Could her "mental age" change for the better?
Yes and no.
She appears for all intents and purposes to be severely intellectually disabled. Unless she's the one in a million "locked in" type who has more mental functioning than their body and neurology can display there is nothing that can make her learn basic math or how to read. She'll never write her memoir via iPad or discuss the weather by tapping at a screen or a keyboard.
BUT.
If Abbie were in a structured environment of any sort, she would be significantly more functional than she is right now. She may be able to partially toilet train. She'd be able to communicate better - no, she wouldn't speak, but she'd be able to use sign more effectively, maybe use her iPad to say if something hurts or ask for something other than food. She may be able to do a few basics of her own self care. With proper structure and oversight she could prepare some very basic foods (no cooking, of course, but assembling a sandwich from pre-portioned ingredients for example).
If her parents had any concept of consistency and structure and routine whatsoever she would probably be doing all of the above by now.
If she were living in a quality structured group home or other professionally staffed facility I honestly feel that we'd be
shocked at how much she would improve.
Again - no, she wouldn't read and write and speak, she wouldn't cook herself some spaghetti - but with structure and consistency she'd be able to do a hell of a lot more that she currently can.
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Yep, that's what I figured.
Same place Brandy works at.
Same place Summer works at.