Give me the Death Note, I can be trusted to act rationally. The only “undeserving” to end up on it would be people who step through Metro doors and immediately stop, even though there’s people behind them.
More seriously I know the author has said it’s about power corrupting, but if that’s the story he wanted to tell then he should’ve chosen a different protagonist. Its also a silly mantra, we do not live in a Tolkien novel, power isn’t some supernatural force.
There’s no disease you can only get when you’re old, people get arthritis at the age of 25. Old age just makes you more likely to get a bunch of different ailments, but they are always possible.
And there’s still the rule about not extending the lifespan, so you still wont extend your life, you’ll just die at the predestined point
Plenty of diseases have age-related in the name, and definitionally only happen to old people, with the variant affecting younger people being given a different name and often having a different trigger. If the Death Note respects human understanding of medical science, then if you specify the age-related variant of the disease, you can be confident it’ll wait. If there are any nuances, you can ask the Shinigami.