First, on age- I don't think anyone is stopping you from making a tulpa who's an "adult", regardless of your age. At 16 I created tulpas who were supposed to be adults.
I do however agree that your tulpa will be limited by whatever flaws you have, but there's a catch. A tulpa may decide some of your flaws really bother them and they will be motivated to challenge those flaws.
Consider myself and Ranger. I am an introvert, and honestly I'm fairly content having lots of alone time. I do struggle with loneliness, but I'm not super eager to go out there looking for friends, aside from having close friends. Ranger on the other hand is far more socially needy, and this doesn't work for her. As a result, she has spent time looking for ways to interact with people. Ranger isn't doomed to being a hermit because of my lack of social skills, and as a bonus she's shown me the importance of socializing with people.
While tulpas will start off with your problems, they can choose to do something about it if they're incompatible with who they are as a person. (edited)