The particular claim I was responding to had assumed that Ingebrigtsen is at least 3 secs faster over the 400 than he was at 16. For any highly trained athlete - and especially an age prodigy, as Ingebrigtsen was - that is extremely unlikely. He had been training seriously and competing before puberty. I compared him to Wilson to show how unlikely it is for an athlete who has matured young to carve 3secs or more off their best sprint time in their teens (and the 400 is a sprint). Wilson won't - and neither did Ovett and Cruz, who both ran 47 at 17.
Your comparison with other teenagers isn't relevant because of the fact that - unlike most of them - Ingebrigtsen has been training like an elite since puberty. He is not a relative latecomer to the sport. In contrast with endurance, speed is the factor in an athlete that is least amenable to big improvements through training. Sprinters make only the most incremental gains once they have begun serious training, and most of it is achieved through honing their technique.
Jakob is not a naturally fast athlete. It is obvious seeing him race. He has very little acceleration to call on and no real "sprint gear". It is one of the reasons he doesn't bother with the 800, in which he is relatively mediocre - and certainly for a top md athlete. I would bet the house, the car and the dog he is nowhere near as fast over 400 as most he assume he is. As another poster above suggested - 49-low at the best.
If he was capable of 48 he would be able to run sub-1:43 for the 800. He would have shown that at some point in his career - as Cram did. I can't see it.