What's the difference between "old" and "senior"? Are they interchangeable?

Native speaker’s answer
Rebecca
There is no difference between "old" and "senior" in this context. "Senior" is a formal way to say someone is old. They are fairly interchangeable, but it's better to call them senior citizens when talking about older people. Ex: Senior citizens get a 10% discount for their meals. Ex: He is really old. Almost 98 years old!