Yeah, it was stated that only kings get the herb and then they had a ceremony where he then ate the herb.
He had it during Civil War. At the first waterfall fight T'challa was given a potion that removed the Black Panther powers. He was given the same potion again at the second fight. Each time he required reingesting the herb to get his powers back. T'chaka was at least in his sixties when he died and supplmental material makes it clear he has already succeeded his father as the Black Panther. Likely due to being younger and better suited for the role.
I would say they are about equal. T'challa appears to be more agile, but that could be due to different fighting styles.