User blog comment:Fliu/One Piece Fan Short: Bartolomeo meets Shanks/@comment-24850639-20170822171724/@comment-53539-20170824065031

^Yes, he has had it for years now.

There is a hint he had it when Roger's execution occurred, however, Oda craftily has hidden that part of Shanks' face each time, so we don't know *when* it happened. But for it to be on his flag, it happened when the crew was very young. We know the crew started forming soon after Roger's death because it wasn't too long later he is recruiting Yassop. So for a good 20 years or so at least those scars have been on his face.

My guess would be that Shanks and Buggy were some of the last crewmembers to leave when the crew was broken up, and some time before their captain handed himself over to the Marines he received it.