I got a warning for the same thing and I was new. It was because I was in a PVT and roleplayed that I was 16. My guess is when a model is new MFC is really watching her and making sure she follows all the rules. They relax after you don't have the new model tag anymore. But the warning is just MFC saying "Hey just so you know that's actually against the rules, I know you didn't mean it but maybe you should read the model rules one more time, k thanks."
SPEAKING OF....
here is the model wiki page that has the definitive list of things you aren't allowed to do. This isn't listed but you also aren't allowed to cam in a public place (library, office, etc). You will be temp banned for that if they're paying attention. As for the under 18 rule, you can say "when I was younger" or you can act like a schoolgirl, just don't say that you're a 17yr old
And It says no illegal drugs but people tip for bong hits so maybe just no hard drugs on cam.
Whatever rules you break or are warned about, I'm pretty sure they'll still give you your money. They wouldn't deduct 10k tokens just for a warning. Even if you did something like scammed hundreds of people and MFC perma-banned you, you'd probably still get sent whatever you earned in that pay period.
PS Read all the threads for new models and stuff for great/amazing/lifesaving tips and advice.