I mean it isn't worth much if they just got immortality surgery. Though I don't believe they're all immortal. At least some of them refer to periods of the void centuries as something they themselves only know as legends. And given we know of other characters way past 100, like Dr Kureha, which is like 140 years old, the Gorosei being very old (especially Saturn, who looks the oldest) isn't yet something extraordinary.
I don't see how the Gorosei are stronger than Kaido. Based on what? Excluding a bit of Haki from their body? Kaido has shown that too. We even know armament can have the same effect (though it should be CoC in Saturn's case).
I don't think they are all from the Void Century, but rather different periods of time. You have to remember, they need the power of the Ope Ope no Mi in order to gain immortality, which is not something that should be easy to come by. It could take decades, or even hundreds years to find the fruit again, once it's been used to perform the surgery. I think only Imu himself is from the Void Century, while the rest of the Gorosei were selected from the most dedicated of Holy Knights, throughout the centuries.
The biggest tell that they seemingly have had the surgery done, is the fact that Saturn doesn't look any older now, than he did 39 years ago. Even in the One Piece World, that's abnormal. Look at Hyugoro after just 20 years. Men seem to be the most affected by the passing of time, yet Saturn doesn't look the slightest bit different after practically 40 years.
And I mean they could be stronger if they are hundreds of years old. That's a long time to be able to reach the Apex of Haki, and completely master their Devil Fruits. Now of course, if they are just regular dudes, who age slower than others, and haven't lived for centuries, then yeah, there wouldn't be any possible way for them to be stronger.