Zoro’s Dojo Master Koshiro is currently 51 years old. The flashback in which Denjiro is fully shown for the first time takes place 41 years ago.
Koshiro would be 10 years when this took place. And Denjiro looks like a 10 year old boy here.
Could they be the same person? It’s possible. Now let’s look at the facial features and the timeline
Same facial shape. Same long hair (ponytail). And both wear sunglasses.
Also, Wado Ichimonji?
How did Denjiro end up in East Blue? And If he is Koshiro, why did he change his name?
THE TIMELINE (Theory)
25 years ago – Oden set out off Wano to join the Roger Pirates. Denjiro asked Oden to go along with him.
Denjiro then later decided to have his own adventure and ended up in East Blue.
22 years ago – In East Blue, his daughter Kuina was born.
20 years ago- Denjiro heard (maybe from one of Oden’s retainers?) that Kaido invaded Wano, and then he himself headed back to Wano.
Oden and his retainers got defeated. Orochi and Kaido took over Wano country.
Then Denjiro decided to go back to East Blue as he had his family there.
He later changed his name from Denjiro to Koshiro to hide his identity (same as what Ashura Doji did).
*Theory by Brix