I enrolled my son at 18 months as that was his first paid seat rather than as a lap child. It was to Australia which was far too long to have a toddler be a lap child on a flight.

More than 2 decades later he is at 0.3MM. He has grown far more than that but most of his flights were award flights.

Enroll as early as possible, although 7 days would be too early for me. His first flight was at 4 months old to Australia as a lap child. He still fitted in the bassinet at that stage. All in coach for those early years but United was good about not putting anyone next to me in the middle seats in the bulkhead. Those days are long gone.