A fully functional lab grown brain would BE a human regardless of how it would be treated.
If the baby or the human had a sufficiently damaged or deficient brain it WOULD be a pile of meat. If they weren't treated as such it would be because of an emotional connection as opposed to a logical reason. This is all you've posited in this comment. Emotional situations designed to confuse the issue. I am using my brain, specifically to cut through the fog.
lab-grown full function brain, would it be treated as a human?
lab-grown baby with brain development stopped at day x, would it be treated simply as a pile of meat?
adult after accident left with x% brain function, would it be treated simply as a pile of meat?
Just use your brain, you will see it is not that easy.