To test my hypothesis, I used the Chi-Square Test for Proportions. I chose this statistical test because I am comparing two proportions - proportion of 30s weekend riders and proportion of 20s weekend riders. My data is categorical. Either a rider is in their 20s or their 30s. Either a ride occurs on a weekend or a weekday. Because my data is categorical and I am comparing two proportions, the Chi-Square test for proportions can be used. The Chi-Square test is also the test that was recommended by my peer reviewer, Nathan Caplan. 
The Chi-Square test was the best test to use, but in order to use it correctly, I had to reformulate my null hypothesis slightly. The null hypothesis for the Chi-Square test for proportions is, for example, "the two sample groups are independent" according to this resource. Therefore, in order to complete my statistical testing, my null hypothesis was reformulated to be "the proportion of people in their 30s who use CitiBike for commuting is independent of the proportion of people in their 20s who use CitiBike for commuting."