I'm looking for an expression that I can use to express "back in the day, when I was young, we used to...". Something like the German damals. I'm not sure if I can use pasintece or maybe tiatempe or something similar. How would you translate it?
This is perhaps more a question about English, but as a native speaker, I'm not sure what "back in the day" adds to the actual meaning of your example phrase. Why not just say "when I was young, we used to..."? I'm more used to hearing the phrase "back in the day" used by itself. (Back in the day, we didn't even have indoor plumbing.)
I would just say kiam mi estis juna or kiam mi estis malgranda. You could also say pasinte or antaŭe if you wanted to be more vague, or en tiu tempo, kiam mi estis juna if you wanted to be more wordy.
I think Pasintece... ("In the past...") should be fine. Antaŭe is "formerly" and tiutempe is "at that time"; tiatempe is more like "at those times".
More ruminative versions are Malnovtempe... ("In the old days..."), Tiuepoke... ("At that point in history..."), or En la iamo... ("In days of yore...")
En tempo jam de longe pasinta is an opening to a Fabelo, but more often those confine themselves to throwing in iam.