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Just another fun part of getting older, as people age many of them start to lose their hair.
Why?
It turns out it's complicated, otherwise we would have fixed it by now, because it's basically the Holy grail of Beauty, but lots of different factors affect whether or not you end up with a shiny bald head. But mainly it has to do with two things: hormones and genetics.

Hormones are involved because they affect how your hair grows. More specifically you can blame testosterone, because balding happens when hair follicles turn it into dihydrotestosterone or DHT. Scientists don't know exactly why, but DHT makes certain hair follicles start to shrink. As they get smaller, the hair gets thinner, and eventually the follicle stops producing hair entirely. Males typically have plenty of testosterone, which explains why balding affects biosex males more than females.

In what's known as "male pattern baldness" hair loss starts at the temples, and the top of the forehead, and at the top of the scalp; eventually the receding hairline meets the growing bald patch. 
But biosex females produce testosterone too, just a lot less of it and they make enough estrogen to mask its effects. After menopause, estrogen production slows, but they're producing about the same amount of testosterone, so that's when the hair starts to get thinner. In "female pattern baldness" it's more of an evenly distributed thinning of the hair across the scalp without much receding hairline.

So how do you figure out whether hair loss is in your future? Well that is more complicated and there's a lot that we still don't know.

Mainly it seems to be tied to genetics and researchers have pinpointed a few different genes that might be involved. Some of these genes are on the X chromosome, which explains why males are so much more likely to go bald than females. Males get only one X chromosome, so whatever version of the gene they inherit on that chromosome is going to be expressed.

Females, on the other hand, have two X chromosomes, so there is more of a chance that they'll inherit a non-balding version of a gene from one of their parents. But studies have shown that there are other genes on non-X chromosomes involved too, like one genetic variant on chromosome 20, which made males 7 times more likely to go bald, but for some reason doesn't seem to influence whether females experience hair loss.

So there are a lot of different pieces to this balding puzzle, which is why we haven't figured it out yet, but if you don't like the new look, there are some drugs that can help. Finasteride, for instance, works by blocking production of DHT. And minoxidil both increases blood flow to hair follicles and widens them, helping hair grow. Of course you could also just embrace your new-found resemblance to Jean-Luc Picard; he did pretty well for himself.

Thanks to our Patreon patron Daver for asking us this question and thank you to all of our patrons, who keep these answers coming. If you'd like to submit a question to be answered you can go to Patreon.com/SciShow; and don't forget to go to YouTube.com/SciShow and subscribe.
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