Props on getting a seat separate seat. IIRC correctly as long as they are under 2 years of age, they could either at anytime. Once over two years old as you note they need to be in separate seat when required.

That said, there are not really any guidelines: https://www.delta.com/us/en/children.../infant-travel

If the infant is under 2 years and asked by a FA why the seat is empty and they are in your arms, just let them know the infant is under two.