My 6 year old calls me Mom or Mommy. We call ourselves Mommy and Daddy to our 2 month old, although Da Da Da and Ma Ma Ma are encouraged as well. And I still call my Mom Mommy even though I'm 27 years old and my younger brother pics on me for it.
But I think she feels special that I still call her Mommy even though I'm an adult now. My sister who is 18 still calls her Mommy too, unless she's in front of her friends or is frustrated then it's just Mom. hehehe.
I kinda hope my little girls still call me Mommy when they're older too. Because as my Mom says they'll always be your little ones no matter how old they are.