'n is the Afrikaans indefinite article, meaning a / an.
A few important points:
So:
A useful writing note: 'n is normally written with a lowercase n, even if it appears at the beginning of a sentence.
Eeu means century.
So:
This is a common way to talk about age in Afrikaans.
Because oud is functioning as a predicate adjective, after the linking verb is.
The pattern is:
So:
This is normal in Afrikaans, just as English says:
Good question. This is a very common point for learners.
When old comes after the verb, Afrikaans uses oud:
But when the adjective comes before a noun, it is often ou:
So:
This is one of those forms you simply get used to through exposure.
A simple learner-friendly guide:
gebou ≈ khuh-BOU
eeu ≈ EE-uh or EE-oo, depending on how fine-grained you want to be
So the whole sentence is roughly:
That is only an approximation, but it is a useful starting point.