My issue is this: I do not know at what age the ability to prioritize is developed(this may be ignorant). I feel prioritizing and weighing pros vs cons are both much more valuable than delaying gratification.
I am not a young child, anymore, but I could imagine a child (me) taking the marshmallow that is sitting in front of me, because maybe I'd rather have a marshmallow now, and waiting 15 minutes is just too long for a second marshmallow. Maybe I'm wrong and I was just a kid who didn't think about candy all the time.
My point is even when I was really young, I still may value my time (not consciously), and would rather go play with legos and have a marshmallow, than spend my time concentrating on getting a second.
Regardless of the accuracy of the predictions.