آپ see "time" is made up سے طرف کی humans. It doesnt exist as some sort of immaterial thing (in fact there is a physical evidence that time doesnt even exist). Why? Cause time is just a measurement of changes and interactions in processes. Its just like measurements of length like centimeters, inches, foot, meter... And dont think of time like clocks and calenders, it is not.
Confused? Let me put it this way: When every single thing in your room were completely frozen, would time be useful? 1 AM, frozen 2 AM, still frozen 3 AM, still frozen .... and time goes one but there is no change, so time doesnt matter to anything in the room. No changes, time meaningless
I know i got it too far but to go back to younger age is to reverse all the changes, interactions in processes that have made آپ become who آپ are in this age. And thats certainly impossible...
Princess Bubblegum can modify her age سے طرف کی adding یا taking away her candy biomass; and since her flesh is made from bubblegum/candy, it is quite easy to do so. However, this way is impossible in real life. But, there may be other ways to do so. We just need to wait.
Since we are in the 21st Century there might be a way to make people younger like what happened to PB. But I think it would be quite impossible to do that in the present time, unless scientists have created a way to make yourself younger.