Null Hypothesis: People born after 1980 are less likely to use CITIbike on weenkend than people born before 1980 or same as people born before 1980.
Alternative Hypothesis: People born after 1980 are more likely to use CITIbike on weekend than people born before 1980.
Since I set the significance level as 95%, if the absolute value of z is larger than 1.96, I will reject my Null hypothesis.