why do people say a man's 'prime' is in his early twenties, but a woman's 'prime' is an entire decade later?

There are different "primes" in life. Sexually speaking, a male's peak desire hits at around 17 while a woman's hormonal apex happens at around 35. But in terms of earning potential most people of both genders don't hit their professional strides until closer to 40. The prime of cognitive faculties tends to be about 35-65 for everyone. If you're speaking purely about physical beauty, that's so subjective that it not worth talking about because different people hold different opinions. 

I think that refers to levels of sexual activity, and that the gap is probably more than a decade. 

Way back when I was 23, I was with a woman then 51 for whom, let's just say, once was nowhere near enough. As the guy in her bed, that suited me very well then. Now in my 50's myself, I know I could not possibly keep up the same level of activity today. 

A man’s prime is typically from 15 to 25 years of age in terms of sex. A woman’s prime is typically 40 to 50 years old in terms of sexual interest. In terms of looks the prime for a woman is probably 25 to 30 years old and for a man it is usually somewhere between 40 and 70 years old

a man will have the most testosterone in his 20s, while a woman's testosterone contunues to increase into her 30s and 40s 

You've got yourself in a snit over nothing. Supposedly that is when a woman's libido peaks, nothing more - like DCMD says. And it wasn't true for me. I am a woman and my libido peaked in my teens and has steadily decreased over the decades.

Because they are completely different primes.

A man's prime at 18 is about physical condition.

A woman's prime is about her being adult enough to have a baby. Human brains typically reach maturity when you are 25

That's referring to the time of life when LIBIDO peaks, nothing else.

If taking into account ONLY that factor, the best matches would be between teenage and early-twenties males, and late thirties-early forties females.

But as joyfully celebrating of sex as I am, this supporter of the Sexual Revolution can see there are clearly other factors involved,  because those such matches are not that common. But it sure explains why teenage and 20-something boys are often frustrated and baffled when dealing with girls in their own age group.

And remember that generalizations are not descriptions of all specific incidents but of general trends. I'm a male in my 60s and I had a massive surge of hormones or something in my 50s, cuz I could *not* get enough. And menopausal women, or -post or pre-menopausal women, might get super agitated and aroused all the time, or forget even the *meaning* of the word "sex."

While yes, there are general trends, humans have wide variations, and if you understand math you know that many generalizations tell you little about individuals you might meet, or even about what's average in the group of people you run into.

It's hard enough to interpret such things, and make rules of conduct for yourself so that you don't waste all your time. It's all hard as hell, humans are complicated critters, and you've made things even cloudier by throwing sexist prejudices into your approach.

If 49 people gather in a backcountry graveyard for a funeral, that already has 50 graves from a small, isolated town accumulated over a century, you COULD say there's a hundred people there, and 51 of them are dead. OVER HALF are dead??? It sure SOUNDS like the scene of a massacre.

But it ain't.

That's how all the "math" of these gender-bigoted "ideas" based on personal pique  always seems to work. And that's regardless if it's men in general or women in general being put down by the particular idiot spouting the nonsense.

Women peak much sooner than men do both physically and as far as earning potential goes. 

Biology....................

No one says that, Joe. A woman's prime is from her late teens to her mid twenties. 

@DMCD: Hormonal decline says otherwise. lol

@Anonymous: It wasn't true for you because it's not true. The testosterone gain women experience as they get older is not enough to overcome the estrogen loss they experience. Due to this fact a woman's sex drive on the hormonal level decreases as she gets older. 

And before someone says it, both estrogen and testosterone play a part in sex drive. For women estrogen is a much bigger player than testosterone throughout her life.