There are exceptions to every rule, and exceptions don't make the rule. Whatever I said applies to Lucci and Kaku, not everyone else in the world. Law at 26 hit the peak of his devil's fruit potential while WB probably didn't do it We all know some people achieve some things later than others. What Kaku's doing is learning the same things that Lucci's been learning and is consistently outdoing him at the same things while still having just as much room to grow further.
Lucci's potential is directly stated by the manga, a comparison like you are trying to do isn't enough to counter that due to their training not being the same.
Age means nothing due to a crazy range for characters peaking or even starting to realize their potential since it relies on outside factors. Oden never leaving Wano would not have been anywhere near as strong as he was when he died for example or leaving years earlier would have made him peak earlier etc
While it's true that Lucci and Kaku are easier to compare because they went through the same training and started at the same age(a fair assumption if not stated anywhere) you are not comparing them at same points/after the same training. You are comparing a Kaku post Water 7 growth to a pre Water 7 growth Lucci(W7 growth is a whole discussion on it's own) and a post CP0 training Kaku to a pre CP0 training Lucci and are just using their age.
While I agree that Kaku keeping pace atleast somwhat unlike the other CP9 is atleast something as is the fact that he got to these opportunities at a way younger age than Lucci even though they had a same start point but I don't think these two things on their own are enough to counter such clear manga portrayal of Lucci's potential.
However that all only applies if the question was simply - who has greater potential between them based on the manga so far?
Within the context of what you said to start this discussion however I think it's fair to say Kaku has greater potential(or atleast arguable) because in this scenario Kaku achieved awakening in 2 years of getting his DF which people arguing with you are ignoring