Edit: Feb 24, 2016 - This page has been rewritten with better information, please click here.
As Tumblr has made DD/lg more well-known and popular, I feel there are many newbies who coming in with the wrong idea of the DD/lg dynamic. I've noticed that a lot of newbies think that DD/lg is simply calling someone Daddy/Mommy and having age regression interests such as sippy cups, coloring books, pacifiers, etc. A lot of them have no idea that what they're interested in is ageplay and not the DD/lg dynamic but if you try to correct them, they'll snap at you with some response about kink-shaming and political correctness.
What is the DD/lg dynamic?
I won't go too in depth in this because I've already covered it in this post. The DD/lg dynamic is a part of the D/s (Dominance/submission) dynamic and is a blanket term for anyone who follows the dynamic and identifies as a Daddy. It is a lifestyle. Mommy, little girl, little boy, babygirl, babyboy, etc. This means that there is a Dominant figure and a submissive and there is a power exchange between the two. The power exchange does not have to be a TPE (total power exchange), it varies between couples - some littles like their Dom to have a lot of control over them while others prefer to only have their Daddy/Mommy control certain aspects of their life. In DD/lg, you'll notice that the Dominant is the Daddy or Mommy and the little is the submissive but sometimes you'll see littles in a Dominant position because they're a switch.
What is ageplay?
Ageplay is a fetish that involves some amount of roleplaying, it's very similar to pet play in the sense that you're acting out specific roles. This means that a person has an interest in regressing or roleplaying a younger age and this can be achieved through dressing up or taking part in younger hobbies and interests. Ageplay divides into three parts: adult baby, little, and middle. AB/DL (adult baby/diaper lover) is a type of ageplay but sometimes tends to be more of a lifestyle for some people. The age range tends to cover baby to toddler age. The "little" division tends to refer to ages 4-10, give or take. Middle refers to the preteen and teenage years, some people do refer to the teenage years as being "big".
Why the confusion?
As you noticed above, a lot of ageplayers refer to themselves as littles or babygirls/babyboys, just as little submissives do.They are two separate kinks though; not all ageplayers are interested in a D/s dynamic and not all little submissives are interested ageplaying. You'll even notice that there are some DD/lg groups on Fetlife that are for age regressing littles and non-ageplaying littles.
You'll notice a lot of people get defensive about terminology and their relationship. "Each relationship is different, based on the needs and desires of the people involved in them." While this is true, there are also titles and groups for a reason - for like-minded people to connect with each other. You are not a DD/lg dynamic if there is no power exchange between you and your partner because of what I explained above. It's okay to be an ageplayer and not have an interest in a power exchange relationship, it's honestly not for everyone. But to tell people that a D/s dynamic is not required to identify as a DD/lg is misinformation and it also puts members from either side in risk of being abused or mislabled as abusive.