Responses
Term of respect used, not only for older people, but for someone whose qualifications supercede your own, or who have worked at a particular company longer than you have. With use of the term, the older office worker acknowledges that his younger colleague has skills he, himself, lacks.
Since Japan edu system has 6 years of elementary school and it looks like they used to be together at school, when Aki started junior high, Saku was 2nd grade? Meaning that their age difference is max 5 years. Since Japan has 4 years of Uni, meaning that Aki wasn't around in Saku's high school era and Saku just got into Uni means Aki just started working for a year? And he's a senpai already?
.........don't mind me, I was wondering why his old looking colleague calls him senpai. Meh.