British women in their twenties have smashed the glass ceiling and are now being paid more than their male counterparts – reversing the traditional "gender gap" in pay for the first time. This is the only age group for which this is true.

According to the Office for National Statistics, women aged 22 to 29 now earn 1.7 per cent more than men in full-time work. Last year men in their twenties earned slightly more – 0.7 per cent.

Overall, and more than 40 years since the original equal pay legislation, the gender gap in pay has fallen to 10.2 per cent this year, down from 12.2 per cent in 2009, and the lowest ever. In 1971 men were on average paid 36.5 per cent more than women.

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The ONS also revealed astonishingly low pay rates among thousands of workers not covered by the minimum wage, either because they are "trainees" or employers ignoring rules.

Even on official figures, as many as 30,000 workers receive less than £2.90 an hour, the equivalent of £116 for a 40-hour week. In all, some 271,000 workers are paid below the appropriate national minimum wage rate – up 34,000, or 14.3 per cent on 2009. Although not revealed in the official data, some of this may be down to employers reclassifying their youngest workers as "trainees" and thus evading the minimum wage. There is also a much larger shadow of informal sweatshops outside the law, and defying health and safety rules as well as employment law.

The young are especially badly paid: the number of 15- and 16-year-olds earning less than £3.57 an hour, the minimum for that age bracket is up by 1,000 to 15,000 this year – leaving them with a gross salary of £142.80 for a 40-hour week, at most.

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The social revolution taking place among the nation's twentysomethings is being driven by a rise in the number of women attending university and their subsequent entry into the better paid professions, notably the law: around 63 per cent of solictors under the age of 30 are female.

But female success in their twenties is not sustained because many leave employment to start families and do not return to work, or at least do not return to the same sort of full-time employment at the same levels of pay.

Recent legislation protecting post-maternity employment rights may erode the still marked differential in male and female pay for those further on in their careers. Full-time working men in their thirties are paid 2.9 per cent more than women, and this rises to a hefty 16.1 per cent for men in their forties.

Recently, recession-driven trends in the jobs market have also narrowed the traditional gap between men and women. Last year was the first where women overtook men in part-time pay, and the differential has widened again this year – to 4 per cent from 2.5 per cent. The slump saw many traditional well-paid jobs in sectors dominated by men, such as construction and manufacturing, disappear, and men have faced the same sort of casualisation in working conditions that women have known for many decades, and seem to have come off even worse.

The drift towards part-time and temporary working, much accelerated in the downturn, has hit men harder than women, the ONS reported.

The decline in private sector pay compared with the public sector has also been a factor in pushing women's pay generally closer to that of men. There are twice as many women in the public sector as men, in sharp contrast with the private sector, and their ability to win higher pay awards has helped narrow the gap.