I analyzed Citibike data in New York City to assess whether there is statistically significant difference in the proportion of young riders compared to old riders in daytime versus nighttime. A chi-squared test of proportions was performed on Citibike data for July 2017 consisting of 1,735,599 rides. The share of riders over 35 years old of all riders was compared for daytime rides (5 A.M. to 11:59 P.M.) versus nighttime rides (12:00 A.M. to 4:59 A.M.). The null hypothesis that older riders make up a greater or equal proportion of riders at night than during the day was rejected at the 95% confidence level with a chi-squared test statistic of 1471.5. The same test was performed on February 2016 Citibike data consisting of 560,874 rides, and the null hypothesis was rejected at the 95% confidence level with a chi-squared test statistic of 503.0. This suggests that the share of younger riders tends to be higher at night.