According to Oda,
Roger would've been 77 currently
current flashback is happening 44 years ago
77-44 = 33
Shanks is 39 and became a yonko 6 years ago (chapter 957) with a missing arm limiting his potential
I don't think this chapter Roger was in his prime just yet (I think he hit during/just after GV) , but he wasn't some kind of rookie that barely started lmfao, he had over a decade of piracy under his belt and could use advanced COC and COA.