Isn't age simply the number of years that have passed since your birth?

It is, but this is such a reductionist, materialist view of the human condition that only bad techbros up to no good would want to talk about it.

Of course Age is about your temporal identity and the intersection of it with social structures of discrimination and oppression. This important fact is not yet well-known, but worry not, soon we'll have a field of study and several journals dedicated to it.

Age is a useful construct for humans. It's an easy to communicate concept based on a shared wall clock we can reference. It abstracts away a lot of more complicated and nuanced factors.

People can accrue all sorts of physical changes at different rates (that still fall within a tight distribution). Some people mentally mature faster than others due to experience, biochemical differences, etc. Some people biologically age faster due to genes, stress, injury, illness, or disease. The volume of UV exposure one receives changes your appearance.

Age happens to closely correlate with so many societal concepts we care about: schooling, maturity and behavioral expectations, dating and marriage, career, having children, getting cancer, dying, etc. Age is universally useful and interesting to us.

That's not how categories and social constructs work. These categories (age, sex, gender) are words that are subjectively defined by us based on utility and common consensus, but nevertheless refer to hard objective reality. Someone with age 35 cannot say they're age 5, because the definition of that particular construct precludes that. The construct points to the objective reality of time elapsed since birth, which can't be changed by your whims.

Agree for age and sex, but gender? Most of us came from a society were there was only two, but ask any hindu about Hijras? So for you Hindus were wrong, but how can they be? Or this is just o society thing (like the color blue being attributed to men since the industrial revolution, and to female aristocrats before).

Yes, even gender. Gender is just another subjectively chosen category, which points to objective reality. In this case, it points to internal brain states, instead of sex chromosomes. It's like the word "Belieber". That's a category which refers to self-described fans of Justin Bieber. There is nothing factually incorrect with such a categorization. In fact, categories themselves logically can't be factually incorrect. It's only the faulty application of an existing category that can be factually incorrect. Categories can have fuzzy borders, but that's often just the nature of words whose definition emerges from conversation and common consensus.

> So for you Hindus were wrong

Hindus were also right -- in their local context where the word "gender" is defined that particular way. There is no globally right or wrong way to define the word "gender", because "gender" is just a word whose definition is decided by local common consensus, just like any other word in a language.

Different societies have different words for common greetings with slightly different implications on meaning. None of them are right or wrong because these words are simply containers that can hold whatever we want them to hold. And it's up to common consensus to put stuff into those containers.

So, this is a social construct then. Since it depends in the society you're in?

The underlying reality doesn't depend on the society you're in, just the categories and definitions of words that we create to make sense of reality and chunk that reality into digestible abstractions. So it's weak social constructionism, yeah.