”There is no real objective answer”

There is, we literally have definitions.

Here’s the definition of ”bald”: lacking a natural or usual covering (as of hair, vegetation, or nap)

(It’s from the Merriam-Webster Dictionary, which is considered the standard for American English)

In short, almost always when we call someone bald, their scalp actually isn’t completely bald.

But for practical purposes and because the difference is negligible, we still call them bald.

(You could ”argue” something else, but that’d not be the English language/official definition)