There is no diagnosis for this behavior. Adults try to act and be like a teenager for any number of reasons. Some people feel they missed out on being young and try to recapture that time period when it’s long past an appropriate time for it. Some people are afraid of adult responsibilities and therefore act overly young so others won’t ask more of them than they can handle. Some liked being a teen so much, they can’t seem to let it go. Etc. There are any number of reasons for not being age-appropriate, but without talking to her I can only give you some generalities.
I’m curious, though. Why is this your concern? As long as her supervisor is satisfied with her work, how she acts shouldn’t affect you. Take the high road. Deal with her as an adult, regardless of how she acts. If you have complaints about the quality of her work or if her behavior makes it difficult for you to do your job, then that’s something you should take up with your supervisor.
I wish you well.