先輩 and 後輩: seniority distinct from age
Distinguish seniority in school, organizations, or activities from actual age.
Someone with earlier experience
先輩 (せんぱい) refers to someone senior in a school, organization, or similar setting. 「大学の先輩に相談しました」 means consulting someone senior to the speaker at university.
「会社では先輩ですが、私より年下です」 says the person is senior at the company but younger than the speaker. Seniority and age need not match.
Someone who joined later
後輩 (こうはい) refers to someone junior in the same school, organization, or experience. 「後輩に仕事を教えました」 means teaching a junior colleague the work.
「後輩から新しい方法を教わりました」 says the speaker learned a new method from a junior. A 後輩 is not always the learner.
Which relationship defines seniority?
「部活の先輩」 identifies a senior in a club activity. The preceding expression narrows the school or organizational context.
「先輩と後輩が一緒に練習しました」 says seniors and juniors practiced together. The sentence alone does not establish that there were only two people.
Must a senior colleague be older than me?
Show the answer and why
No. Seniority at the company and age are separate criteria.