When the film came out I was, I believe, 14 years old. And looking back today, my impressions of the story are very different from those times.
At 14, I identified with Juno. I thought Vanessa was a bitter woman and that Mark didn't have space to be himself. I thought "wow, she leaves all his things in one room".
Today, as an adult, as a mother, I see Juno as a typical teenage girl. And I was shocked by how much I understood Vanessa. Mark wasn't a poor thing, but an adult who didn't want to grow up. Vanessa had no intention of erasing who he was, and even found a room just for him to have space to put his things, and have a space of his own in the house. Mark wasn't ready to truly commit to the situation. He wasn't a father yet. Vanessa was already a mother. Even without a baby. She had motherhood inside her and was more than ready for it. She was not a bore, but an adult. But in the eyes of a teenager it is difficult to understand an adult. Maybe Vanessa also liked listening to different bands, but she was past the moment of wearing band t-shirts around. After all, she doesn't need it and it won't make her more of a fan or not of the music, she doesn't need the t-shirt to establish who she is - because she already knows who she is.
Not to mention Juno's family arc, which is very important in her choice of the couple, and also in her expectations.
The film brings all these reflections and more, being a very true portrait of human issues.