We don't know Xebec chronology, but I agree he wasn't far from prime during that time imo.
We don't know precisely, but imho It has always been clear he was of an older generation compared to Garp/Roger. I have been saying this since before the FB even began.
I was surprised some people thought he was of the same age of Roger and WB, even though mostly It was because of agenda and just to push the rookie Xebec thing.
Anyway, Roger and Garp are ~ 40 at GV, so Xebec should be ~ 50. I would bet on ~55 since that tends to be the the age of narrative prime for quite a lot of top tiers (Roger, just to say one). So, 40~ at Marijoa.
Also, Xebec objectively looks way older that 20 at Marijoa (if he was of the same age of Roger, he would have been 21 at Marijoa). And Xebec wasn't 20 at Marijoa lol, even though I shockingly saw people saying he didn't look older.
And he didn't even look mid 20; he looked 30 at least.
There are other considerations, but these are the most relevant.