Ex: She is the young girl that he met many years ago when he was in a prisoner of war hospital.

The word that jars is ago where the time clause is past tense.

before - at some unknown time before now.

ago - at a certain time before, counting back from the present

Another way of writing the above sentence with before would be to make use of a past perfect tense:

Ex: She is the young girl that he had met many years before when he was in a prisoner of war hospital.

Even if the lapse of time had been specified, the sentence would still call for before:

Ex: She is the young girl that he met twenty years before, when he was in a prisoner of war hospital.

Ago is for counting back from the present. Therefore, when time is specified, ago is in use.

For variation, you can use earlier or previously to indicate a time before a time already in the past.

Source: Anhvangiaotiep