I heard that after 70 years old, we have only bonus years. What is the source for this?
One vision of the average lifespan is recorded in Psalm 90:10
The days of our years because of them are seventy years, and if with increase, eighty years; but their pride is toil and pain, for it passes quickly and we fly away.
ייְמֵי שְׁנוֹתֵינוּ | בָּהֶם שִׁבְעִים שָׁנָה וְאִם בִּגְבוּרֹת | שְׁמוֹנִים שָׁנָה וְרָהְבָּם עָמָל וָאָוֶן כִּי גָז חִישׁ וַנָּעֻפָה: