There's no proving speculation right or wrong, hence no point in debating it till the story gets to a point where we have facts to deal with.
What happened to him was more than him turning into an old man. He became a sack of meat unable to move a muscle. Luffy wouldn't need G5 altogether to fight Usopp, and even if he decided to use it, it wouldn't be needed for long enough to put himself into the state he ended up in against Kizaru.
Lucci was case in point for that. It was a casual, decisive, one-sided victory, even though Luffy suffered from the side effects of G5.
He ended up in the state he did against Kizaru because he had to retain G5 for beyond his limit in order to land a hit, so yes, Kizaru did indirectly cause it, while a weaker character wouldn't have.
If you have to literally knock yourself out in order to defeat an opponent, that's no victory at all.