I think what your friend means, is pretty much the same thing that could be applied to
any supplement. In some cases, taking certain drugs supresses your body's natural production (this is more related to medical conditions or drugs which would require a prescription, but not always).
In some cases, your body gets lazy and relies on the supplement/drug so your gains/losses are artificial and misleading because "you" didn't technically achieve them, the drug did.
In some cases, the drug (or supplement, all the same really) will help you achieve results prematurely (but a better example of that would be steroids), and when you make gains unnaturally fast, it means the rest of your body cannot adapt to these changes and injuries or damage can occur. (Very slight examples of this are: like when a woman gets pregnant or people pack on fat or muscle very fast and the skin can't adapt slowly, and you get stretch marks,.... or growth spurts in teens when the bones are growing at what is sometimes a painfully faster rate than the neuropathways and loss of balance occurs, or taught
connective tissues,....or vocal chord development & voice changes).
I know, you asked about creatine, but those (above) are some examples of what I am guessing your friend was refering to about changes your body can go through when being changed through quick means.
As for creatine, like many other supplements, you will (or many do, anyway) experience changes, such as increased power in your lifting or sprint. You (body and mind) will become accustomed to what this feels like, emotionally and physically, and when you go off of it, you won't function or perform the same way. That's because the gains/power you felt were because OF the creatine in your body. So you may find you've lost something, the edge perhaps, in your lifting when you go off of it.
That's been my experience, just one other voice in the crowd, and I know we all feel and experience things differently.
Another opinion would be: At 16 you may want to wait a while before
taking creatine or any other supplement, and work very hard at seeing what your body can naturally develop into on your own, and under your own power first. Without a strong baseline, naturally, you're cutting yourself off. (You know the saying, cutting off your nose to spite your face kind of thing)
<<Did I say that one right?>> heh