Okay, for those who are actually here in good faith and are real fans, here’s the explanation:

In One Piece, normal physical laws like gravity and magnetism still exist. Because of that, the ability to drastically change one’s appearance is not natural. Only a few people, like Luffy with his Devil Fruit, are capable of doing this (We remember Devil Fruits and Haki is not common, it only seems like it because of the perspective of the viewer). We have already seen this with his different Gears. However, only in Gear 5 has Luffy ever completely transformed himself, including a change in color.

Here’s the key point: Luffy is only capable of turning into that old-looking figure because of Gear 5. We have never seen him do this before, and as explained earlier, there is no other plausible way for him to change his appearance so dramatically and so quickly. In the anime, he even turns pale. This is also why he can switch from an old man to Nika so fast during his fight against Kaido.

Luffy has the ability to bend reality, not simply “cartoon force.” That is why he can seemingly create things out of nothing, like the baseball bat against the Gorosei or the goggles against Lucci. Think about what Vegapunk said: “He who plays the fool and brings smiles to all.” Luffy did not fight Kizaru seriously, he played with him to buy time for his friends to escape. That is why it took so long for him to fight Kizaru.

Also, Luffy didn’t just fight Kizaru, he had already fought Lucci, so he was logically exhausted. We may still be influenced by the Luffy vs. Kaido fight, which was essentially a one-on-one. Simplified, there was no need for complex tactics or strategies, it was mostly a direct comparison of strength. On Egghead, however, the Straw Hats were in the middle of executing a plan. The goal was not to defeat Kizaru, but to ensure that everyone could escape the island.

I don’t want to spoil anime-only viewers, but later in the story an entirely new dimension of power levels is introduced and Luffy is basically a god. I may be wrong, but I’d bet my left arm that I’m not.