She was always shown to be a mature person. She took to being a superhero very easily, and was thrown into the deep end as an Avenger almost immediately, which didn’t seem to phase her.
As to her and Adam as a couple? I’m a fan. It feels like a natural pairing and not forced, like a lot of these relationships in comics. They compliment each other and make for a formidable power couple. A couple that, like Adam himself, has been downplayed instead of played up.