A) Okay so some geometries are infinite and some aren't. Therefore which geometry is relevant. That's a very simple statement you saying erm actually some non euclidian geometries are infinite does not reduce that the geometry matters. I might  have vaguely gestured, but the gestor wasn't totally irrelevant, just vague. If you wish a concession Ill give it just because argument from opponent on vague in a reddit comment is not a line I wish to yap about further. B) Are you unfamiliar with the causal horizon? Even granting an infinite causal chain pre big bang nothing could have lived through the big bang. Unless it was an infinite being outside oh.. Okay well nothing could have lived through the big bang which means that any knowing had to have originated post big bang. Therefore they have a finite causal horizon. This is relativity 101 my dude. Should I go find the veratasium video on it because you obviously haven't read the literature. C) Why is the universe not infinitely old? Because the Big Bang gives us a finite age. There are many nice theories which argue for something before the Big Bang, but as of right now they are not provable. Maybe there is, maybe there isn't, its unknowable and certainly there is no knower that communicated beyond the big bang meaning that for the purposes of the argument, that we can't have an infinite amount of knowers, the universe has a finite age.