According to the theory of time travel commonly depicted in science fiction, if a person were to travel back in time, their age would not automatically reduce based on the number of years they are going back. Time travel would typically involve moving through time while maintaining one's current age and physical state.
For example, if a person is 30 years old and travels back 10 years, they would still be 30 years old but would find themselves in a time period 10 years earlier. Their physical appearance and age would remain the same. So, in this scenario, the person would not become 20 years old upon traveling back in time.
Additionally, based on the concept of time travel as commonly understood, it is not possible to travel back in time beyond one's own birth. The reason is that traveling to a time before one's birth would create a paradox, as it would potentially lead to a situation where events occur that prevent the person's birth in the first place.
It's important to note that time travel is purely theoretical at this point, and there is no scientific evidence or consensus on its possibility or mechanics. The above explanation is based on the commonly depicted notions of time travel in science fiction.