Very simplified
The whole body is made of cells. Theses cells contain chromosomes. Chromosomes hold all the information of the body. Bits of DNA gets copied and processed by the cell to make everything form cell walls to proteins and then the chromosome splits/divide to make new cells and restore itself by replacing existing cells or grow.
This happens constantly, eventually telomere gets damaged and shorter. But the important DNA is intact. Once the telomere is gone the DNA receives damage and it causes ageing.
We’re not talking about maturation and mental aspects. We’re not even talking about configuration, organization of the cells. We’re talking about DNA repair so that you cells can be healthy, reproduce and produce protein normally. That’s what we mean by young.
And your DNA will be set back to a time before the damage, 16 years old.
Same brain, same way, but inside the cells, the DNA will be healthy.
That’s just one small thing in the field though