"lead" or "drive" covers most of its meanings. It really is somewhat of a generic verb, similar to and often translated as "do". You won't see it very often in Wheelock's except for in phrases such as
gratias agere and
vitam/tempus/aetatem/etc. agere. In
gratias agere for example, you could think of it as meaning "to do thanks"; "to give thanks" is a less literal translation refinement. In the phrases where
agere takes some amount of time (vita, tempus, etc.) as a direct object, it is usually best to translate as "to lead/pass ______ (e.g. a life, a year)".
Wheelock's gives about five different definitions for it and in one exercise 'age, age' means 'come! come!'.
This imperative form is not from the Latin
agere but from the Greek equivalent. Latin has adopted the imperative form of the Greek word which explains why the meaning is so different from that of the corresponding Latin form.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae