He won’t define the relationship.
Immature guys love “situationships” because no title gives free-reign or any type of behaviour or effort he feels like displaying on a whim. He will not want to define a relationship, because this will hold him to standards and responsibilities to live up to.
He can’t hold down a job for long.
A mature man will have his finances in order and is willing to work hard to maintain his financial security. This is harder for an immature guy, who is flakier and less reliable in many aspects of life. If he can’t hold down job responsibilities, he will likely not be able to hold down the fort of your relationship.
He cannot or will not express his feelings.
Many guys are more of the “strong, silent” type, but it is healthy and critical to express feelings to your partner in a relationship. This may not require many words for a mature guy, but he will be able to articulate his feelings effectively nonetheless. An immature guy will not know what he feels, or just be largely against tapping into his emotional side at all. If he’s super immature, he may even get annoyed or angry when you ask him to express himself.