Its fairly common practice in games to avoid any possible way to deliberately kill children. In some games, like Fallout or Fable, children are just invincible, while in others, mainly a lot of sandbox games like GTA, the world is just mysteriously empty of them (so you don't end up killing any in the havoc). This is generally because the ability to deliberately choose to kill a child character (as opposed to an adult character, or a child dying via storyline) is more frowned upon by the ESRB and tends to warrant higher age limits in games.
While there is no indication that it was necessary, Terraria notably has a unique death message and "gore" for the Angler; instead of saying he died and falling into body parts like other NPCs, it says "he left" and he disappears in a puff of smoke, sounding like a Wraith dying. Thus you can't "kill" him like other NPCs, he only leaves.
Its unlikely the game would actually get an increased age limit due to such a small oversight, but it is worth reporting and getting fixed, if only for consistencies sake.