Citi Bike is a bike sharing system which is widely used by people in New York. It is important to analyze the riding patterns in order to provide better services. In this analysis, I investigated the age groups of night ridership. Using Z test and Chi-square test, the proportion of younger riders biking at night was compared with older riders. The tests results show that younger people are more likely to use the services at night hours. The analysis can facilitate the allocation of bikes at night hours.