It would be difficult to grow one’s hair Head hair wouldnt get long, if it were curly. Curly hair has an oval cross-section. Long hair has a round cross-section.
That said, my Dad—as a boy—had straight hair. Returning from Navy PBY service in WWII, his hair had turned curly.
He never mentioned this phenomenon.
Beard hair is triangular in cross-section. FWIW...