Lots of headcanon to when he even learned it.
We’re not even sure if this was really kid Shanks that Oda was talking about. For all we know, this can be years down the line and some notable members saw up and coming Shanks doing Roger’s moves.
15 year old Shanks might have been way stronger than a F6 already, who dictated that Shanks hasn’t unlocked acoc by this time? Divine Departure is most likely Roger’s own signature acoc attack. Basically, the nuances of when Shanks learned it is up for debate. What’s not up to debate is that Oda explicitly stated that only Shanks can even mimic it, Rayleigh is of no exception to this as he is the most prominent/strongest Roger pirate as I’ve said. The natural guy you’ll think of when talking about talented Roger pirates that can potentially use Roger’s moves.
Well, if we're going that route, we may as well assume he learned it at 37—which doesn't sound quite right but still within the realm of possibilities. When the information is limited, you go with the most likely scenarios, not the far fetched ones. So it's natural for the people to assume Shanks tried copying Roger when he was still around., which is why vast majority of us assumed a 15 year old Shanks learned it.
It's also, not very likely that he was way stronger than the F6 although it's not impossible. If he was, then it's pretty damn impressive as even PK level potentials like Luffy, Ace, etc, were nowhere near that level at a similar age. Once again, Rayleigh isn't the only skilled Roger pirate. He legit represents their peak after Roger. Even current Shanks gets hyped for being called stronger than Rayleigh. He's not your average standard.
