I've done the 'Older Woman' threads to death on this forum, but since I'm in a writing mood today, I'll bite.
I've tapped my share of older women when I was in my 20s and I can tell you that the sex was no more or less extrordinary than the younger women I've been with. The only major difference? I never had to worry about 22 y.o. single girls finding a babysitter for a night or had to be concerned with making too much noise during sex so as not to wake up her son in the next room. Nor was I concerned about it being "her weekend" to have the kids. If the common belief is that older women encourage an idea that they are more sexually available, you've got to ask yourself, why would they be motivted to be more so at 35 than 25?
Don't get me wrong, there are many attractive women in their 30s & 40s but these are uncommon exceptions to the rule. The social reinforcement of the "older woman" fantasy is just a modern extension of the "she's still got it" social convention with the latent purpose of leveling the playing field for 30-40 something women unable to sexually compete for the same calibre men with 18-28 y.o. women.
Women's sexual value naturally declines as they age - it serves an older woman's purpose if she can redefine sexuality as her conditions change through life, and convince society that she's correct and genuine. Ashton Kutcher and Demi Moore may epitomize this fantasy, but in reality, there are thousands of women filling gyms across the country for every Demi Moore convinced that they "still got it" while every year a new crop of 22-24 y.o. hotties commands the attention of the same men they're competing for. This is just the natural extension of the 'Have It All" lie that women have been sold for the last 50 years.