I didn’t realize that one had to be less than some age to be eligible to vote. The rest of the exchange is the ramblings of someone with a slight (maybe not so slight) mental deficiency, but this age assertion is even more bizarre.
I think the foot-shooting implication by Joe was either that A) old people won't support "new ideas" or B) that the heckler will be dead by November 2020. If Biden had been Hillary, I'd put money on B.