Sascha Zverev, Nick Kyrgios, Dominic Thiem to name a few. They are rising stars at the ages you talked about but expecting them to win slams at this age doesn't make sense because the sport has CHANGED dramatically since Chang and Samparas and Becker won as teens. The game is way more physically and mentally demanding now.Where are the rising 19, 20, 21 year olds? Pete Sampas won his first major at 19. Jimmy Conners won his first major at age 21. Bjorn Borg was called "Teen Angel" at age 17. Micheal Chang won the French at age 17.
Make no mistake - the rising stars are there , but teenagers winning slams will be unlikely in this era of tennis.