There is no shortage of data about our ageing population. If the government's consultation on reforming the care and support system in England ends up going nowhere, it won't be for lack of facts and figures about present and future numbers of over-65s, over-85s and over-100s (a staggering 57,000 by 2031, it is projected), their life expectancy and, more pertinently still, their healthy life expectancy.
As is reported elsewhere in Society Guardian, we are also getting better at quantifying demand for services such as mental health care. The numbers may be worrying, but at least we are developing a clear picture of the scale of the challenge.
There is, though, one sector that remains by contrast a statistical black hole, and that is learning disability. In a report today from the Centre for Disability Research (CDR), the country's top experts admit: "It is not possible to estimate the numbers of adults with learning disabilities in England either from information held by central government departments or from large-scale, population-based surveys."
This brutally honest assessment is startling enough in isolation. We have put man on the moon, we can watch television on our mobile phones, but we do not know how many of our neighbours are learning disabled. When you add a financial context, however, the lack of knowledge starts to look not just neglectful but grossly irresponsible.
If you include the Supporting People programme, learning disability accounts for almost a quarter of all personal social services expenditure. What's more, this £4bn spending is soaring: last year, more than eight in 10 councils with social care functions identified it as a cost pressure. Survey figures for 2007-08, due out shortly, are expected to show that independent nursing support for people with profound learning disabilities is the single fastest rising cost across adult social care, up no less than 13% in a year.
The growth of personal budgets is exposing the breathtakingly high costs of some care and support packages - as much as £400,000 a year in exceptional cases - and while no price may be too great to pay for independent living or the best achievable quality of life, these are numbers that decision-makers must have fully in their sights.
Hertfordshire council has rather more learning disabled residents than average, because of resettlement from local long-stay hospitals, but is otherwise not untypical. Sarah Pickup, its director for adult care services, says she has had to increase learning disability spending this year by 7.9% in cash terms. "So we are having to put in nearly as much for learning disability as for older people, despite the volume of demand being so much greater for older people because of the demographics."
What do we know? Today's report, commissioned by the charity Mencap, repeats the CDR's earlier estimates that one person in 50 has a learning disability and one in 200 uses learning disability services. This gap is explained in part by the unknown numbers of learning disabled adults who are cared for by parents themselves becoming frail and dependent.
One reason for the spiralling costs of services is that such families are emerging from the long grass in need of support. Another, happily, is that learning disabled people are living much longer lives. A third, and again we need to find out much more about this, is that there is evidence that severe learning disability may be up to three times more common among children and young people in Bangladeshi and Pakistani communities than in the general population.
The government is absolutely right to extend its consultation across care and support for all adults, rather than just older people. But, as the Learning Disability Coalition is asking, how can we make sensible judgments about the future when our knowledge is so poor?
· David Brindle is the Guardian's public services editor
