There’s been a lot of discrepancy regarding Thirteen’s age. In actuality, we have enough information to figure out how old she is. Based off the information we have though, there’s really only one age Thirteen can actually be.
    
We know from House’s Head that Thirteen was not born twenty-eight years ago. Therefore, she is definitely younger than twenty-eight. Younger than twenty-eight is a big catergory that gets a lot smaller when you consider that Thirteen finished medical school. Assuming she didn’t skip any grades in school, which is possible, and didn’t graduate early, also possible, she has to be twenty-six to have graduated from medical school. That means that she has to be at least twenty-seven now, because if she was twenty-six, she would have been twenty-five and still in medical school during the fourth season.
    
When you think about things that way, she has to be older than twenty-six and younger than twenty-eight, which means Thirteen has to be twenty-seven.