It is not often, in these dark times, that one stumbles across a snippet of good economic news. So it's strange that one such shaft of sunlight in the gloom has gone mostly unsung. According to official statistics released last week, the pay gap between men and women – that barometer of shifting power between the sexes – has quietly shrunk to a record low and among younger women has shot clearly into reverse. Women in their 20s now earn a solid 3.6% more on average than men their age, after narrowly overtaking them for the first time last year. The rise of the female breadwinner, it seems, was no blip, but the beginning perhaps of a social and sexual sea change.

For an angry but vocal minority, that is a change too far, yet more proof that they are the underdogs now, trampled beneath the stilettoes of supposedly over-mighty women. The conservative family policy expert Jill Kirby even suggested that "the pay gap we should be worrying about is the one that shows young men falling behind", not the one that still sees men earning more than women for every other decade of their working life.

Losing ground is admittedly never easy, even if that ground wasn't always earned, as one glance at the Tory backbenches, boiling with resentment at young women being promoted over older male heads, confirms. The trouble with shattering the glass ceiling is that someone inevitably ends up ducking the flying fragments.

But it's worth remembering that, barely a century ago, the great male fear was not of alpha females with intimidatingly large salaries but their polar opposite: women were seen, rather like immigrant labour now, as dangerously liable to undercut men's wages by doing the same work for less. Equal pay was sold not as a threat but, rather intriguingly, as a promise.

As the then mayor of New York put it in 1911, explaining his decision to grant 14,000 female teachers the same salaries as men: "Instead of lessening the number of male teachers this will increase it" by removing the financial incentive to hire women. Even in 1946, the Royal Commission on Equal Pay set up in Britain argued that equality would mean women losing their jobs, since "at equal pay for men and women, a man will always be preferred". Why on earth would you hire a woman, unless she was going cheap?

Half a century on, it seems incredibly mean-spirited not to allow young women at least a moment's triumph over proving such arguments wrong, before making them hang their heads in shame for the men they have left behind. But instead, the same reproachful message has been drummed into them since their teens, when they outstripped boys at GCSE and A-level only to face howls of protest about education being rigged in their favour.

It's not that this argument was without any merit. Where boys are failing, schools should question what's happening in the classroom. It's just depressing that the debate so often contrived to make young girls' strengths – greater social confidence and maturity or a conscientiousness that makes them better at coursework – sound strangely like cheating, since these skills have turned out to be undeniably handy.

After all, those same girls went on to beat boys at degree level, to form the majority of trainee barristers and solicitors and fast-track civil servants by the middle of the last decade. They're the same girls who, a graduate recruiter once told me, shone so much at interview that they left the boys standing.

And they grew up into the same junior managers who, according to a recent survey for the Chartered Management Institute, now out-earn their male counterparts for the first time since 1974. Even if the pay gap between senior executives still yawns so wide that the CMI estimates it will take a century to close, they must have been doing something right.

What is emerging now is a striking generational divide. The pay gap for full-time workers is biggest now for women in their 50s – those least likely to have been encouraged when young to pursue a career or hang on to one after children. But it narrows with every decade subtracted from a woman's age.

Roughly speaking, as girls' horizons have widened and their skill sets swelled (only a quarter of girls went to university in the 1960s, for example, whereas by 1996 they outnumbered boys), their earning power has risen in tandem. Legislation, industrial action, a greater emphasis on traditionally "female" skills, such as communication, and sheer bloody-mindedness all helped.

But one reason young women now get paid more than their mothers is simply that they're worth it, a basic fairness that matters more to a cohesive society than perhaps we used to think.

After all, what fuels the festering anger at rocketing boardroom pay isn't just naked envy of the 4,000% increase in some top bosses' salaries over the past three decades, as the High Pay Commission reported, but a feeling that there's no rhyme nor reason to it. It's not as if their companies are thousands of times more profitable. And this breaking of the link between effort and reward is a profoundly unsettling thing: why strive to do your best if you get nothing while the undeserving merrily trouser their bonuses? Which is precisely how too many women have long felt about their male colleagues.

It's easy to forget not just how stonkingly, grievously unfair things have been in the past, but also how tentative these female gains have been. Men who work full time still earn 9% more than women overall, hardly suggestive of being chucked on the scrapheap – any more than the existence of a measly five female cabinet ministers (outnumbered five to one by men) really spells matriarchy.

We don't even know yet whether this is merely a case of tortoise and hare, with young women shooting off to a confident start only to find themselves overtaken the minute they pause to have babies. After all, men's earnings start to outstrip women's from the age of 29 – precisely when Mrs Average now has her first child.

It's far from clear that this generation of golden girls can beat the so-called "motherhood penalty", either by managing hitherto unsuspected feats of juggling or by persuading some of their lower-paid husbands to take on more at home. But at a time when hopes of future wage growth for anyone seem few and far between, perhaps we could at least stop hounding them for trying.