Hmm...
My mother never let me call her "Mommy". She doesn't like that word (something about that y-ending diminishing or infantilizing her...?), and so she taught me to call her "Mama". I guess if she HAD to be referred to as a mother, she wanted something that sounded more dignified (to her). I started calling her "Mom" in later elementary, but she doesn't seem to have noticed. In fact, when she's relating something I've said to her, even today, she'll start off with, "My daughter said, 'Mama, etc..." I don't call her that any more, but she apparently thinks I do.
Personally, I have no particular preference, but my kids used "Mom" and "Dad" pretty much from the start. I think there might've been a brief period of early toddlerhood when it was more like, "Mama" and "Dada" but that was before they could talk well.
To me, it's not a matter of growing up quickly or not. And I don't see any particular significance to the choice of word. You have to call your parents something!
I like being "Mom" to my kids.
Side note: My mother also refuses to refer to me by my actual name. See, when I was born she wanted to name me a common nickname, but felt she couldn't actually name me that. So she gave me my real name and then set about calling me by that nickname. Unfortunately, she failed to fill everyone in on her plan when I started school, and, having learned my real name, I quickly decided I preferred it and would go by it. She's now the only person in the world who insists on calling me by that nickname. Hilariously, when my son was born, I decided to name him a different common nickname. My mother said, "You can't officially name him that! That's a nickname! You have to give him a proper name and then everyone will call him by the nickname." I just looked at her and said, "And how did that work out for you?"
