What age was Bruce Wayne (Earth-1) when he left to go train.
Do you mean Earth-One the golden Age Earth or the current Earth?
Or the current Earth-1/Earth 1 from the Earth One graphic novel series? If the latter, if it's not in any of the issues it's unknown.
I would assume once he hit 18 years of age he left
In Batman: Year One, he was 25 years old when he came back to Gotham. Seven years training seems plausible
What do you think?