* 後輩 (kōhai): This refers to someone junior to you in a company, school, club, or other organization. This is probably the closest general equivalent.
* 下級生 (kakyūsei): This specifically means a junior in school (like a younger student).
* 年下 (toshi-shita): This means "younger" in terms of age. It's less formal and doesn't necessarily imply a hierarchical relationship.
* Adding -さん (-san), -くん (-kun), or -ちゃん (-chan) to a name can sometimes indicate junior status depending on the relationship. However this is not a direct translation.
To choose the best word, you need to consider the situation. If you're talking about a junior colleague at work, 後輩 (kōhai) is the most appropriate. If you're talking about a younger student, 下級生 (kakyūsei) is better. If you're just talking about someone younger than you, 年下 (toshi-shita) works.