Your "seniors" (although not used very frequently) are people who are simply older than you.
"Senior citizens" = elderly people (in the UK, anyone over pensionable age, which is 60 or 65, can be referred to as a senior citizen.
My boyfriend is 20 years my senior = My boyfriend is 20 years older than me.
"Senior" is used in job titles to describe someone as a higher level (Senior Sales Director, for example), but we wouldn't just call them "a senior".
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