CPU cores have a temperature near 80 °C and then the fan starts to work. Is it too late?

I have MBP 13, 2015.

When I watch some videos CPU Core 1 and 2 have a temperature near 70-80 °C. At this time the fan starts to work with some 1300 rpm.

Is it a bad temperature in the meaning of long term condition for the cores' life?

Should I configure my fan to increase the rpm at lower temperatures?

No, it is not a "bad" temperature. If your MBP 13" 2015 model is configured with the default CPU, it is specced to work at up to 105° C. So running at 70 or 80 degrees is no problem at all.

In general you should never change the configuration of your fans away from the defaults supplied by Apple.