There are a few exceptions where you actually put the adjective before the noun and "vecchio" is one of them. Here is a list I found for you on the internet :
bello (beautiful) ; bravo (good, able) ; brutto (ugly) ; buono (good) ; caro (dear) ; cattivo (bad) ; giovane (young) ; grande (large; great) ; lungo (long) ; nuovo (new) ; piccolo (small, little) ; stesso (same) ; vecchio (old) ; vero (true). There might be more but as you can notice those ones are very frequent and very short adjectives, probably why there are before the noun.
I know that this is a forum for learners of Italian, but for those of us who are not native English speakers it also provides an excellent opportunity to brush up our English. So I wonder if anyone could explain why the noun is preceded by the preposition 'in' instead of 'at' at the end of the sentence? Is there difference in meaning or are they enirely interchangeable?
I am not an English major (far from it!) but I am guessing there is a subtle difference between 'we are arriving' and 'we arrive'. "Hey Rowan, when will you be arriving at the port?" "We are arriving right now." Or the other option, perhaps more subtle. "Hey Rowan, when will you be arriving at the port?" "We [will] arrive at 9:00 pm." In other words, one is more an event occurring right now, and the other at some time in the future. Some English major may tell me I'm totally off base!
Your example uses "the port" (as in, we arrive at a specific port), not a port like the Italian sentence, which is more vague. So "we arrive in AN old port" is less likely to be used to describe a future action than "we arrive in THE port of Dover tomorrow". Although it is possible. E.g. "Where will you end up tomorrow?" "We arrive in an old port". Even then though, I'd be more inclined to answer with "We'll be arriving in an old port" or "We'll arrive in an old port".
However, this sentence is just isolated without any question inferred. And as a single statement without any other context, "we are arriving" still sounds better to me than "we arrive". "We are arriving in an old port" makes it clear that the arrival is in the process of happening right now. Whereas "We arrive" sounds more habitual. Although it could indicate a future action. But without further context, it's impossible to tell. So really, both forms of the present tense should be accepted here, not just one.