I heard the same thing too! Preparing you for Adulthood!
I was also told not to drink cold water after exercise, the cold water would cause your stomach to rupture. (I have yet to ask the internet about this... Anyone know if its true?)
Doubtful or there would be many dead. CamelBak reps made a big deal about drinking cold water while engaged in activity in hot climes. Their caution was cold water 'doesn't quench your thirst' as well as tepid. But they had no answer for why they had expanded their fill spouts to enable the use of ice.....just like all the boys fighting in Iraq did when they could get it.