Each Red represents a new timeline
Classic Red
Advanced Red
MEGA Red
Go Red

Now you might say, but hold on advance Red and MEGA look similar enough for it to be a simple age progression.
Sure but Advanced Red plays out parallel to RSE and we know Brendan from advance looks totally different than the one from Oras.
Their eyes and skin dont even match.

And no advanced doesnt play in the same timeline as MEGA. RSE didnt had war 3000 years ago (ORAS) and no they dont have infinite energy either (ORAS).
Let's go Red obviously had his journey before the creation of the Pokedex unlike classic and Advanced.
Classic Red has Espeon and Advanced has not.
So as you can see there are many differences.