User blog comment:Jademing/Oda is terrible at math/@comment-4783583-20120305211149

Roger's execution was 22 years ago, brook knew about roger from 50 years ago.

50 - 22 = 28

Meaning that 28 years before becoming the pirate king Roger was a rookie.

My guess is that Roger was about 47 years old when he died meaning that

47 -28 = 19

he was nineteen (luffy's current age) when he made a name for himself as a rookie.