The calculated Z statistics is 58.61. The largest number reported in Z table is 3.5 that gives a p value of 0.0002, which is smaller than 0.05. The results show that we can reject null hypothesis with a p value close to 0. The proportion of older riders biking at night is significantly smaller than the proportion of younger riders biking at night. 

The calculated chi-square statistics is  3434.787, which is much larger than the threshold 3.84. We can reject the null hypothesis. The proportion of older riders biking at night is significantly different from the proportion of younger riders biking at night. 

Then I used the dataset from July 2015 to verify the results. Null hypothesizes were rejected with Z statistics and chi-square statistics equal to 91.31 and 8338.16, respectively. The results are robust that younger riders are more likely to use Citi bikes at night in warmer weathers as well.

The test results show that younger riders are much more likely to use Citi Bike services at night hours than older riders. This information is useful when allocating the number of bikes. For example, we can expand the number of available bikes at the stations near the neighborhoods where have more young people at night hours, such as the stations near universities in New York City.