Not _quite_, although it's a fairly pedantic technical distinction.

As I understand it, an organism that reproduces sexually (rather than asexually) has a sex dependent on the gamete it produces. There _might_ be some insects with more than two sexes[1], but generally a dioecious (two-sexed) species will be anisogamous, meaning one sex is responsible for bringing a load of smaller, simpler gametes (sperm, pollen) to the party whilst the other brings a single, larger one (egg, ova). Put them together and babies happen.

The mechanism that determines the organism's sex varies by species. Homo sapiens are a species that have an XY sex-differentiation system. As a sexually dimorhic species (our two sexes look different), one's chromosomes also cause one to express distinct secondary sexual characteristics (e.g., male facial hair, female breasts). Man boobs and bearded women exist, however, so these characteristics are not a 100% accurate indication of one's sex.

Also, as another reply stated, the meaning of ‘gender’ may or may not == that of ‘sex’.

Gender referred only to a linguistic concept or was a synonym for sex until the '60s-'70s, when radical feminists and sexologists like John Money[2] proposed that gender be used to refer to the expectations, roles and responsibilities that a culture assigns based on sex (i.e., women should raise kids, men are more aggressive). Personally, I feel a term like ‘tertiary’ or ‘socio-sexual characteristics’ would have been a better, less confusing choice, but unfortunately I wasn't around in the '60s to tell them.

Outside of social sciences, the words generally remain synonymous. Within social sciences the '60s-'70s-era distinction is retained and gender is studied almost exclusively (as you might expect of a sociological concept rather than a biological one).

[1] https://www.sciencedirect.com/science/article/pii/S016953470...

[2] https://en.wikipedia.org/wiki/John_Money

How many people are there in the world for whom you could say, with reasonable confidence, that you know what gender they are? (This is not a trick question.)

How many of their chromosomes have you seen?

Can say the same thing about age, no?

I was gonna say, I have a far _far_ harder time determining my associates age than I do their gender. But that's no surprise, seeing as how only 4.5% of adults in America identify as LGBT; in other words, the vast majority of the people you come into contact with are straight, and generally conform to the stereotypes we've come to expect.

That would be biological sex, not necessarily gender.