In chapter 966, it was mentioned that Shanks have been an intern for a long while when Blackbeard was asking about him and Buggy.
When Roger was holding Hiyori, he mentioned that it has been a long while since there’s a baby on board. Same for Rayleigh. It means that there was indeed a baby on board before.
In chapter 964 when we see Shanks and Buggy reading newspaper, Shanks was 9 at that time and he already had the straw hat from Roger. When did Roger give it to him?
During the fight at God Valley 38 years ago, we could see that Roger doesn’t have the straw hat anymore.
38 years ago, Shanks was 1 year old and it fits the idea that Roger’s crew had a baby on board before.
Exactly where did Roger pick up Shanks? It could be at Lodestar Island as it was mentioned that it was an event 13 years ago during the flashback which Roger mentioned. It is therefore exactly 39 years ago when Roger landed on Lodestar Island which is when Shanks was born.
But Shanks was stated to be born in West Blue, how did he end up on the New World? Perhaps this is the mystery why Shanks seem to be always appearing in random places and why Roger wants to pick up him.
*Theory by boyplunger56