It's Toshishita Jidai

Aoi is a high school student who belongs to the astronomy club. She has been wanting to confess her feelings to Egawa, a senior who is about to graduate, but has been unable to do so. However, her days of mixed feelings of anxiety and anticipation come to an end when she hears the other person's unkind words. Aoi is depressed, but then her childhood friend Shintaro, who is two years younger than her, appears. As a noisy junior, he makes Aoi laugh and annoys her ......

(Source: sukima, translated)