When you want to.
Anyone can make ANY rules. That literally takes no effort.
The problems are when you want OTHERS to accept them. Now that takes waaaaaaay more effort than most people expected to or wanted to, and in most cases will have no satisfying results, as the efforts will not be rewarded by others. So if your goal is dependent on acceptance, then it'd better that you waste it on more beneficial ventures.
Again, its the same thing for whether you're making TRPGs, foods, arts, science, cultures, religions, politics... The fundamentals are the same even if the goals aren't.
Yes, making rules and making it acceptable to others, are two separate things altogether. v=3=v
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Modding an existing rule, on the other hand, isn't the same as making one up from scratch.
You're just picking up other people's effort to make it fit you better.
Most everybody do that more or less. You're pretty much expected to do this eventually.
But that doesn't make you a "creator" per se, just saying.