Final answer:
The Roman law that differentiated between juveniles and adults and prohibited capital punishment for those under the age of 20 was the Lex Pompeia.
Step-by-step explanation:
The Roman civil law that created the distinction between juveniles and adults, and which stated that no capital punishment should be given to those under 20 years old, is known as Lex Pompeia. When discussing ancient Roman law and its evolution, one can see how these laws laid the foundations for contemporary legal systems.
Moreover, the Roman law system was the first to introduce the concept that someone accused of a crime was presumed innocent until proven guilty. This legal presumption was a breakthrough in the jurisprudence of the time and indicates Rome's sophisticated approach to law and order.