Weird, I don't think older women are invisible at all, though I say that as a 33 yr old woman not a man. I love older women and have more older friends than young, their wit, personality, life experience, take no prisoner attitude, I love it. If you think attractiveness is just about looks, you're very deluded, I see the way men look at some of these perfectly average looking middle aged women, they're not invisible.
I always remember the time I went on a break to Paris, I was mesmerised by a woman in her 60s/70s, silver hair in a bun, white shirt, khaki combats, black biker boots.
One of my favourite colleagues is approaching retirement, her mischievousness, sparkle in her eyes, I never get bored of her company.