I can say from personal experience that the 25-year rule is a myth. I have, over my life, been a variety of sizes, and have on numerous occasions successfully changed my body's "set point" for the weight it tries to maintain. There was no magic year marker where it solidified into some immovable thing.
Now mind you, how easy it is to change this depends on the person (for lots of people, it's harder to change than it appears to be for me), but everything I've seen suggests that everybody can do it, no matter how old they are, it just takes a certain amount of work. (Of course it also does become more work the older you get, so it's better to get it where you want it earlier in life if you can...)
And the number of fat cells you have is really completely irrelevant. Even if you have tons of fat cells, that does not mean that it somehow magically takes your body a different number of calories to gain a pound of extra weight. The whole fat cell "fact" thing that gets quoted so often is generally just a scare tactic or excuse tactic, depending on who's using it and why. It's bad science used for effect.
(It is also technically possible for your body to lose fat cells, but this generally requires being extremely low-fat with a reduced calorie intake for a sustained amount of time, at which point the body will naturally begin to catabolize the fat cells. The problem is that the point at which it does this is also well into the point where it's catabolizing muscle tissue too, so it's not generally a healthy thing to do. As mentioned, since there's really nothing wrong with having extra fat cells anyway, there's no real reason anybody should attempt to do this.)
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Now mind you, how easy it is to change this depends on the person (for lots of people, it's harder to change than it appears to be for me), but everything I've seen suggests that everybody can do it, no matter how old they are, it just takes a certain amount of work. (Of course it also does become more work the older you get, so it's better to get it where you want it earlier in life if you can...)
And the number of fat cells you have is really completely irrelevant. Even if you have tons of fat cells, that does not mean that it somehow magically takes your body a different number of calories to gain a pound of extra weight. The whole fat cell "fact" thing that gets quoted so often is generally just a scare tactic or excuse tactic, depending on who's using it and why. It's bad science used for effect.
(It is also technically possible for your body to lose fat cells, but this generally requires being extremely low-fat with a reduced calorie intake for a sustained amount of time, at which point the body will naturally begin to catabolize the fat cells. The problem is that the point at which it does this is also well into the point where it's catabolizing muscle tissue too, so it's not generally a healthy thing to do. As mentioned, since there's really nothing wrong with having extra fat cells anyway, there's no real reason anybody should attempt to do this.)