I think it is a myth lemme tell you why:
Shoulder length hair is hardest to care for. The ends are brushing against your shoulders all the time which causes friction. You really have to protect your ends.
How do I know?
I was told at thirteen (by ignorant ppl) that my hair would never grow passed shoulder length.
I ended up doing protective styles. Just buns because I didn't know how to braid at the time. And I stopped using heat. COMPLETELY.
Now at 21 after I BC'd in 2009, I have midback length hair. Hip length when straight.
Just take really good care of it while it's at shoulder length.
Mostly 3b with 3a and traces of 3c.
BSL (WL when straight)
Conditioner: Herbal Essences Hello Hydration
Style: Pantene Mousse for curls + Pantene Hair Whip
Shea moisture curl enhancing smoothie
Coconut oil applied on a nightly basis.