That's an old wive's tale. I have two boys 14 and 17. They've both been training with weights since age 12. My 17 yr old is a little over 6'2 and my 14 yr old is a little under 5'10. My 17yr old started doing powerlifting comps at 15 and has been training with heavy weights since 14. He's 195lbs now and one of the top linebacker recruits in the country. My 14yr old is doing his first powerclean comp later this month. He's been banging heavy weight since 12-13. I'm 6'4. And I think they both will be taller than me. BTW, my oldest has benched 275lbs, squatted 445, and deadlifted 525 all Raw and of course natural.