Once again, that answer begs the question as to why gender is a social construct. For many, they consider it to be the same as biological sex; the view that it is somehow determined by society is to them a falsehood - and one that can't be proven wrong, if the criteria are biological. Hence, the idea that gender is a social construct is in a sense a matter of opinion, because there are those who, seeing it as biological, don't hold to that view.
Age is also a social construct, once we acknowledge that it is defined according to a human calendar. The natural world cares nothing for social constructs like a calendar. Furthermore, we are not all the same "age", from the point of view that some mature physically faster than others, and some age faster. Chronological age does not measure those features - which can vary considerably in a way not determined by an antiquated Roman calendar.
We are no closer to understanding why gender is a social construct simply by asserting it as a fact - and saying aging isn't, when we see age is also subject to human conventions. We say something is a social construct because we choose to say it is - and in some instances we can so choose. Death is not a social construct and nor is disease, because nothing we choose to say about them will change the facts of what they are. Gender, on the other hand, is malleable, if we treat it as a convenient label and not an immutable biological fact.