An emotionally immature partner can keep up appearances.
They might have a good job, be overall responsible, and be seemingly well adjusted in society.
Emotionally, it's a different story.
You can't get close to them. They shut down. They blame all issues on you, and rarely reflect on the role they play.
They might even use their status to make you the issue, not them.
Not everyone has the maturity to create an adult relationship. Don't let appearances fool you.
Emotional age comes out when: a person hears no, when things don't go as planned, and when they feel overwhelmed or frustrated.
Pay attention.