Her maternal instinct
As much as some women would like to fight against their inherent biology, they were made to bear and raise children. When women don't have children of their own, they tend to project all their nurturing instincts toward the closest person in need, which, in this case, is you. Not all women are destined to or even want to be mothers, but most women have the capacity and even the drive to take care of something or someone. When this comes in the form of making you soup when you're sick or listening to your troubles after a long day at work, it is most welcome. When it results in repeated requests for you to cut your hair already or pick up your socks, it may bring out the teenager in you.
What to do: When she treats you like a child, you can counteract the negative results of her maternal instinct by acting like a man. She is less likely to think of you as a child if you regularly demonstrate that you are strong, independent and self-sufficient.