Perhaps it’s awarded if the answerer gets a GA on a new user’s first question, before the new user asks their second question.
It could be that once the new user has asked their second question, the Starfish will not be awarded to any subsequent answers in the new user’s first question.
Would that be anytime less than 48 hours after the new user joined, or anytime up to the end of the first day after they joined? E.g., if the NU joined on August 1, then would it be anytime up the end of August 2nd, even if they joined at 11:59 PM on August 1?