Obviously not "all" leavers are old and not "all" remainers young, but there is a very strong tendency in that direction.
Aaranovitch seems right to me. It's highly probably that if the referendum took place in 2020, the result would have been different.
It's also a bit of a useless point, because the referendum didn't take place in 2020, it took place in 2016. So it's 2016 demographics that count. I suppose he could be suggesting we should have another one, but that's fundamentally different. It's not clear at all that Remain voters actually want another referendum to happen - so if it did, it's not very clear how they would vote or even if they would vote in it at all.
So he's probably right. But it's meaningless information anyway.