No, it is not possible for a person to become so young that they shrink down to the size of an atom. The size of an atom is on the order of a few angstroms (10^-10 meters), while the size of a human being is typically on the order of meters. The concept of "becoming young" doesn't involve changes in physical size but rather refers to the passage of time and the aging process.
It's important to note that individuals cannot simply shrink or grow smaller in size without a drastic change in their physical structure or the manipulation of fundamental particles and forces. Our current understanding of physics and biology does not support the idea of a person shrinking down to atomic size while remaining alive and retaining their human characteristics.