This happens in the episode where everyone prematurely ages, and they are sent through the transporter to make them their "normal" ages. There's no reason given why they couldn't do that all the time.
Even more relevant there was that episode where a transporter accident turns Picard, Guinan, Ro and Kiko into twelve year olds and nobody points out they just discovered transporter induced immortality.
What really gets me about that episode is all of the effected characters immediately want to return to their normal age and nobody says "Hold up, I'm very okay with a couple extra decades of life" or centuries in Guinans case I suppose.
Let's not even address the fact that transporter enyouthened Picard would presumably still have his cybernetic heart. Was it an adult-sized cyberheart in his kid-sized chest cavity? Did the transporter know how to resize the prosthesis to fit his changed body? So many questions!