"Fifteen years ago."
Translation:Vor fünfzehn Jahren.
Still trying to remember why sometimes it is Jahre and sometimes Jahren.
It's "Jahren" because plural nouns generally take an extra -n in the dative case. This is dative because of the preposition "vor". In all other cases, the plural is just "Jahre".
I'm still learning myself, but I don't think you can use seit here.
From what I understand, vor is for events that occurred the past and are no longer continuing today. Seit is for events that started in the past and are still continuing today.
For me, Fifteen years ago implies something happened in the past.
If you wanted to say something started in the past and is still continuing today, you would probably use something like ...for fifteen years as in I have been learning (and am continuing to learn) German for fifteen years.