I feel that mostly happens to people who either have weak bodies to begin with, those who might've been in a terrible accident (and/or other circumstances that might lead to these results), or those who don't and/or can't take care of themselves as they're aging. It might have something to do with how you're genes are wired too, but the point is that not everyone deteriorates as they get older.
In my family -- you're only as young as you feel. Everyone takes care of themselves because they want to live as long as possible and have a meaningful life, for however long as they live. They don't try to stop learning, even if some of their minds might slip, and they don't stop being as energetic as they used to be.
Like -- death is inevitable. Shit's gonna happen. So we might as well live life to as full as we can. And that has, ironically, made many of my family members live longer.
I come from a family of people who live into their late 90s, early 100s. And these people did the same thing they always did. Some drank, some smoke, some played cards, some still drive their cars, go out, and have fun. And I'm mostly surrounded by older people in their 40s, 50s, 60s, who are having fun and just...embracing life.
My god mother -- who's 87 -- still makes her moonshine, still lives in her house, and rollerskates. She's in very perfect health. And my great uncle -- who's 96 -- still tries his drive, goes out, is still walking, and lives alone. Of course, we will always check up on them because, despite how healthy they are, something could still happen at that age. But we don't treat them as children because..well, they're mostly doing fine. They're taking care of themselves.
So for me -- birthdays are a celebration of life. You have to make life the way you wanna make it worth while. Like, even if you didn't celebrate it, death is still gonna happen. So I'd rather celebrate the beginning of my life with the people I care about than not to.
Just -- for my end -- life's not gonna stop being meaningful for me after 25. If anything, it's gonna start becoming more exciting! And I'm looking forward to it.