Age for Bar/Bat Mitzvah

I have always understood that the age for Bar/Bat Mitzvah was rabinicaly set, that halchicly the guideline was physical maturity. Now if a law D’rabbanan can only create a fence for a law D’oraita, how could they set an age for Bar/Bat Mitzvah, especially today when we see physical maturity occuring much earlier and thus D’oraita many of these children should be Bar/Bat Mitzvah but are not treated as such?

By Torah law both Physical maturity AND age count, the rabbis decreed that age in it self counts.