Complex sentence: In the legal systems of many countries, there is an age of majority when childhood legally ends and a person legally becomes an adult, which ranges anywhere from 15 to 21, with 18 being the most common.
Split into simpler sentences: In the legal systems of many countries, there is an age of majority when childhood officially ends and a person legally becomes an adult. The age ranges anywhere from 15 to 21, with 18 being the most common.