You can't use it directly in a query because it is an aggregate function, meaning it does an operation on a set of data. So that is max(age) in the context of the sql below?
select * from person where age = max(age)
You probably mean the max(age) every person, but you must be specific on that. So what you can do is:
select * from person where age = (select max(age) from person)
Hopefully the above helps.
Good luck!