Most of us, most of the time, manifest a Soul Age less than our "official" one. This can be a slight amount less (perhaps one Level) or it can be a very great deal less (perhaps 2-3 complete Ages). There are about as many reasons for this as there are fragments doing it, which may be as high as 6 billion.
In all cases, it is done for a good reason, although that reason is unlikely to be comprehended by the fragment. Sometimes, the purpose is one by Essence (perhaps that there was something much earlier in this life that was left unfinished and needs to be completed before moving on) and sometimes it is a move by Personality (sensing that the current situation would be better dealt with as a Young Soul than as an Old Soul). Neither of these may turn out to be the optimum manner of dealing with the immediate situation, but both Essence and Personality always operate in the fashion that they perceive as optimum in the situation as they perceive it (these perceptions often will look rather silly in retrospect, but they always make apparent sense at the time.
I am a 7th Level Old Soul, but I have often been able to look back and see that I was operating in a Young Soul Mode in a particular situation, and it was, probably, optimum for the situation.
Later,
Gordon