Plan Effective date for newly unrelated entities

We have two entities owned separately by husband and wife that are under common control due to a minor child. Minor child will be 21 mid-2021. At that point, ASSUMING no other attribution rules apply for the spouses (no ownership in the other entity by either or management/oversight, not in a common law state, etc.), can a plan be opened after the child's 21st birthday with an effective date of 1/1/2021? Must it have a short plan year or fiscal year beginning after the birthday since the entities were under in a controlled group at some point in 2021?