Well,

It could be more that we lose interest in anime 
about 16-17ish aged characters after awhile ^_^; 
You don't see a whole lot of anime that really 
relate to 40 - 50 - 60 - 70- 80ish adults ^_^; 
Also I know from experience that the 60 - 70 - 80 
(primarily the "white" people though) age group 
tend to be against the Japanese, mostly because 
they remember WWII, so seeing them start 
collecting anime cels is doubtful in my mind.

I still love anime, but I'm even myself 
embarassed that my favorite characters (which I 
have plastered all over my room) are 15 and 18 
years old O_o. I mean Trunks is 23 by the end of 
his saga in DBZ, so at least he's closer to my 
age (27) but Heero is 17 at his oldest in the GW 
series. My non-cel-collector (or non anime fan) 
friends probably think I am a fruit O_o....

Of course, if you read about GW you find out that 
the years AC 195, 196, and 197 are supposed to be 
parallel with our years 1995, 1996, 1997... so 
that means Heero-sama would be 21 now which is at 
least a little more acceptable :) 

I personally plan on being an anime fan forever 
^_^ but I can see how for many people that it 
would lose the appeal. It could be that you get 
to the point where you feel a lot of pressure to 
let such "childish" things go.

I think that's why we will end up having kids ^_^ 
so we can still enjoy watching anime with them. 
It makes it a bit more acceptable :)

-misa