emotionally underdeveloped means, you dont know how to deal with certain situations due to lack of experience, in accordance with your age. For example, if someone who was 27 had have several boyfriends, whereas someone else never had one by that age they would be underdeveloped emotionally in that area. Maybe finding it more difficult to understand the other as they not gone through the feelings and events of a relationship.
You can be emotionally underdeveloped in many different areas though, it hasnt got anything to do with being mature or immature.