OUR perception of old age is changing.

Of course it is. We are living longer, so it's not surprising a survey this week shows that in Britain, middle age now starts around 55 and people don't see themselves as elderly until pushing 70. The exception is television executives, who believe women are over the hill at 50.

The BBC newsreader Fiona Bruce has re-opened the festering sore of discrimination against older women broadcasters by revealing that, at 48, she dyes her hair because it's unacceptable for a grey-haired woman to read the news, although the issue would not arise with a man of the same age.

It is ironic that women broadcasters have coined the term "sageism", with its suggestion of wisdom, to describe the toxic combination of sexism and ageism that many claim forces them off the television screen after they reach 50. This Neanderthal attitude not only means they lose knowledge and experience, but also risks irritating viewers who value professionalism over good looks.

Ms Bruce may have been making a pre-emptive strike in the knowledge that the corporations's new director-general, George Entwistle, has committed to having more female presenters. Until now, British television has been behind the curve. If it holds up a mirror to society, it is a distorting one. In the real world, slowly but surely older women are becoming more visible as they reach senior levels across a wide range of professions.

At 56, Christine Lagarde carries off silver hair with an elegance that has fuelled so many requests for copycat coiffures that hairdressers all over Europe now know the name of the head of the International Monetary Fund. If her predecessor Dominique Strauss Kahn ever rated a mention in salon gossip, it wouldn't be in relation to his hair.

German Chancellor Angela Merkel – arguably the world's most powerful woman as she orchestrates the survival of the eurozone – is 58. Who cares whether she dyes her hair? Hillary Clinton, the US Secretary of State, is more image- conscious but, at 64, she remains resolutely focused on defusing the latest foreign policy problem.

It is notable that in the US, despite an obsession with the body beautiful, older women, such as the legendary broadcaster Barbara Walters (83 next week) and 54-year-old Christiane Amanpour, global affairs anchor for ABC, are valued for the knowledge and gravitas they bring to news and current affairs. That may also owe something to more deeply entrenched age discrimination laws. Age discrimination is illegal in almost all types of employment in the UK, but it is suspected employers are using an exception to support inter-generational fairness to hire younger (cheaper) staff.

High-profile television presenters will easily find alternative employment (Fiona Bruce has said she would like to become a magistrate) but their contemporaries are increasingly on the scrapheap. In the last year, unemployment among women aged 50 to 64 has risen by 16%, but fallen by 1% for men the same age. It may be more misfortune than deliberate discrimination, but women in their mid-50s are also facing the biggest loss as a result of the raising of the state pension age.

New research from the International Longevity Centre-UK shows that an increasing number of older people are experiencing social exclusion, with an unexpectedly steep rise among those in their 50s. Becoming a carer is a significant factor. This means that while hale and hearty 50-year-olds can happily protest they are barely middle-aged, many women in their 50s and 60s, unable to find work but caring for elderly relatives, are on the cusp of poverty and isolation in their later years. Yet they have decades of useful experience, are exceptionally reliable and won't be looking for maternity leave.

The commercial world has recognised the power of the grey pound. Hollywood has finally twigged that people over 50 are sexually active, although it remains to be seen whether the release of Hope Springs in cinemas this week (with Meryl Streep and Tommy Lee Jones as the older couple embarked on sex therapy to revitalise their marriage) heralds a more positive depiction of over-60s in popular culture.

Meanwhile, employers are missing a trick if they fail to recognise the benefits of adding a few grey heads to their young hustlers.