What to do: If your girl is starting to display the negative characteristics she has learned from her mother, point it out to her. If she is one of the many women who doesn't want to end up like her mother, this should be enough to set her straight. You could even agree on a quick code word that you can use when she's throwing a mom fit, like referring to her by her mom's name.
You asked for it
Naturally, another potential reason for her to treat you like a child is because you're acting like one. It's no secret that women mature faster than men, so you and she may not be on the same page when it comes to maturity levels. You may want to pull a teenage rebellion fit when she tells you there isn't enough money in your budget to buy a PlayStation 3, but keep in mind that the more you act like you're 12, the greater the chance she's going to pull out her mom look and stare you down.
What to do: Act your age and the nagging should subside. If you can rationally explain your behavior or desires, do so. If, however, you can't come up with any adult reason for them, just cut it out.
Looking out for you
Whether it comes out as nurturing or nagging, in most cases it comes from a place of genuine affection. If she didn't care so much about you she wouldn't bother to lecture you on how much fast food you've been eating. Just remind her, calmly and without a confrontational attitude, that you are an adult and fully capable of making your own decisions. Add that you very much value her advice and appreciate her help and you'll earn major relationship brownie points in the process. Sometimes her reminders are genuinely helpful; try to see them for what they are and don't overreact.