Interesting :) And good to see there are some other posters the wrong side of middle-aged! I imagine the reason ageism is noticeable on Mumsnet is that we're engaged in conversations with younger adults speaking freely. At work or in Starbucks, they're less likely to rant about the crapness of "old" people while you're within earshot - they'll do it about you, after you've left!!
I was part of a mass redundancy program when I was 48. From my department (of about 50 people), all eleven of us were aged over 40. Coincidence? Surely not ...
Things get worse as people get older, certainly, and women's problems get worse faster than men's. I don't see any point in limiting this discussion to the over-70s, though, for several reasons.
Ageing is different for each generation. My 80yo mother has lived longer than her parents and is aeons 'younger' than her grandmother at 80. My generation is much 'younger' than our parents were at our age. Our old age won't be the same as our parents': we will probably be significantly poorer, but fitter and more mentally agile. (And the British climate will be colder.)
We are the 'baby boomers', our generation causes a population bulge at every stage. We will be the elderly population everybody seems so scared of. It's up to us to deal with that, 'cause no-one else ever has nor ever will.
Our social and economic environment has changed, and continues to change, almost beyond recognition. The Britain in which we are elderly will not be the one experienced by today's 80-year-olds.
Age prejudice kicks in much younger than you expect. If any woman thinks she's immune until 60, she's got a few shocks ahead.
There's an interesting feminist challenge in this: Can women turn their loss of patriarchal value into a strong advantage? If we're becoming invisible, could that lead to a stealth attack? I was thinking not - as we're not only invisible, but also mute - then I remembered the WI 
Everyone forgets it exists, except for the odd joke about cake competitions, but in fact it's a very influential body that can affect government policy.
We need to be far less silent! We also owe it to ourselves to learn more about the diseases of ageing, such as alzheimer's, arthritis, osteoporis, cataracts, sclerosis, etc, and start taking care that their effects are better managed and more widely understood.