Zoro's doji master Koshiro's current age is 51. This flashback is 41 years ago, so he would be 10 years when this took place
Denjiro looks like a 10 years old boy here.
Could they be the same person? It's possible. Now let's look at the
facial features and
the timeline
Facial features
Same facial shape. Same long hair (Ponytail). And both wear sunglasses.
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 asks Oden to take along with him.
He 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 (See in chapter 0)
20 years ago- He heard (maybe from one of the 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 then changed his name from Denjiro to Koshiro to hide his identity (Same as what Ashura Douji did).
Now, Imagine Neku arrives on Wano with Marco and Koshiro (aka Denjiro?)
That would be fucking hyped.