Normally you can take only one Brain Age test per day. The results are saved into your profile. However, if the result (the Brain Age) is not good then any other Brain Age test results will not be saved to the record until the next day.
Use the following trick to redo the test for that day and get a better result without waiting for the next day. After the test, do not save the records results. Press Home to quit and reload the game.
You can now take the test again (with different questions) for the same day. Repeat this method until you get good result.