Bruh how can you gauge someone's age with his looks especially when he looks menancing anyway
And, it's just deduction.
At God Valley, Roger was around 39, BM, 30, WB 34, Shiki 30 to 34 so most likely Xebec was around 40 ~ Roger's age as well because Sengoku referred Xebec as Roger's first and greatest foe
Then, Teach is around 40 as well
If we go back, then he met Loki at elbaph -48 years ago - Xebec age - 29/30 years old.
Reverie incident - 56 years ago - Xebec would be around 22 at that time