These two terms are relative. A person's "seniority" in comparison to someone else is often determined by age, but social status, expertise, and other factors also play a role. For example, an expert in a field of study would likely be addressed as "Senior" by a novice in the field, even if the novice is older than the expert. And in a company or organization, higher-ranking members/employees are typically Seniors while those beneath them are Juniors.
