Oh, I would never in a million years do anything with their in-show ages. I always write them as late-teens, since I have an easier time with that and think it's more interesting when they're aged up.
That said, I completely understand that some people might be like "GYAH MY CHILDHOOD MAKE IT STOP," so there's plenty of warnings and you can always give it a miss. 
Yeah, I think that's what might have caused the mix-up... I don't believe I ever saw their ages explicitly stated anywhere, so I could understand where readers might assume they're the same ages as they were in the show if they aren't familiar with your other writing. Perhaps an additional note to their ages in your warning? I don't remember all that the warning said since I read it a few days ago, but I remember thinking it quite apt for the content, and that you did a good job of it.
I also thought that the story was relatively tame for most 'M' stories you see on fanfiction nowadays. I've pretty much given up on M fictions since most of them are actually MA in content... don't people read the rating guides anymore!? I'm actually considering posting the prologue to my "A Household I.Q. of 782" as an 'M', 'cause by the rating standard, it probably falls under that... but by the general trend of fanfiction, I could probably keep it with the rest of the story at a T rating...
Anyways, I also wanted to say that I found myself sketching Cindy at the part in the story (the one this topic is actually named for, not the fan service one) where she's in her dress and gazing sadly into the mirror... very moving scene, creeping up under my subconscious hand and all... xD