Bailey's design is based on real world people. She is EXCEPTIONALLY shy, and a fairly big introvert. The only exception to this is in regards to the people whom she loves and trusts. For those people, she's incredibly upbeat, perky, excitable. She hasn't been sheltered like Skylar and Kaylee have been, at least not by the actions of her parents. Instead, it's a result of her own personality. She's not one to sit on social media 24/7. She's not a huge TV person. She's a "sit in my room reading John Green books" kind of girl.
Because of that, there's an internal struggle. A natural desire to experience more, to explore the body and follow up on the strange emotions she feels towards her sister's boyfriend (and, to a lesser extent, her sister as hinted at back in the early stages of the game).
So while she may be PHYSICALLY mature, her reclusive nature has resulted in her being sexually, emotionally, and socially, immature.
This is similar to how Skylar is. Due to her upbringing, she had many of the same issues. But since she is now able to explore the world in her own way, and with her own control, she's ready to spread her wings. So long as she feels safe doing so.
Kaylee is in the same boat as Skylar, but far more aggressive about getting off that boat. She's already been testing the limits of what she can or can't do under her parents roof (Pointing back to the scene where she shows off her Skyrim character). Kaylee isn't waiting until she gets out in the world. She knows what she wants to experience, and she'll do it on her terms.
Hope this helps to explain how the characters were designed
