My 18 year old had pulpectomy on tooth number 19, he had decay in the tooth and was in pain, is a crown always necessary after a root canal.

Depends: Depends how big the cavity was, how much tooth structure is left, and how hard your child's bite is.

Root canal therapy: Yes, when it involves tooth #19 or any other tooth that is used during chewing and grinding food. Tooth #19 is a first molar and has a large chewing surface with 4 cusps. When a root canal is performed, the tooth becomes brittle and prone to fracture. That is the reason a crown is necessary; to protect the cusps and envelop the chewing surface of the tooth from forces that could cause fracture.

Almost always: If the cavity did not involve the walls and it was a narrow and deep cavity a crown is not needed, just a filling. Unfortunately almost always the walls are compromised and if this is the situation a crown must be done otherwise the tooth will crack in the future.