If I come to shul early for maariv and it is already past nightfall and there is a minyan that starts davening mincha am I allowed to answer amen to chazaras hashatz? Sources please.
Disclaimer: this answer does not deal with the latest time for mincha. It assumes that the question refers to a time when one for sure cannot pray mincha.
If it's really not the time for mincha at all then don't say Amen as the blessings are levatala.
I can't prove this for late mincha specifically, but by a late shacharit, the Biur Halacha (OC 89 sv VeAchar) says that if someone prays shacharit more than a half hour after chatzot then his blessings are levatala according to many poskim. [Note that in general the last time for shacharit is 4 hours into the day, and post facto until 6. The Biur Halacha here is accounting for a very minority opinion which permits praying shacharit until 6.5 hours.] So we see that praying a certain prayer after its time is considered a blessing levatala.