For those who are confused, Jishin is actually 19 (same as our protagonist mao mao) since throughout the manga 2 years have passed, but he is posing as a 24 yr old eunuch as some form of job from his father, the emperor, which is to weed out any unworthy and unfaithful women in the harem (he's the gardener of the emperor's "flower garden"). From what we know, we can assume that Jishin is actually being forced to do this job since he is the only heir to throne atm (a consort's son at that) hence why in this chapter he was really wishing for the emperor to bear a new son. Back then, the only people allowed to stay in close proximity to the women of the harem were other women and eunuchs (b/c they were educated men who were chosen by the emperor to tend to the women of the harem as doctors. They had to be castrated as a precaution for the women of the harem to not be tempted to sleep with these men and vise versa) Jishin, however, is the only heir to the throne, thus it would be a terrible idea if he were castrated and not able to continue the blood line. So, instead of being castrated, he had to ingest medicine that would allow him to appear impotent without actually being impotent.