Although Rowley was older than White Beard, Rowley didn't eat the devil's fruit. His fighting power comes from the clever use of domineering. Plus fencing and physical skills.
So Rowley's physical fitness is better than white beard.
Secondly, Raleigh has retired and White Beard is still active in the forefront.
As the strongest man, White Beard has many enemies. In addition to the navy, there are other pirates who want to replace the white beard. When Rowley was drinking tea and picking up girls in Shampoo Island, White Beard might be fighting the enemy.
Although the white beard is very strong, it is a flesh after all. In the peak war, he was killed by black beard because of assassination. It is conceivable that even a small-scale war, as long as the white beard is shot, the body will definitely be hurt.
If this goes on for a long time, new diseases and old diseases will not be able to withstand such tossing and turning. Therefore, White Beard's physical condition is not as good as Reilly's.