Thanks for asking this question - it sent me on a very enjoyable little search for info on daily fibre recommendations!
It looks to me like the absolute minimum that the experts say anyone should have is 14g of fibre for every 1000 calories consumed ( www.hsph.harvard.edu/nutritionsource/fiber
), but that 25-35g is considered a reasonable range for any healthy adult. The reason behind the 21g for 50+ women is that it's based on an average, lower calorie diet (because apparently we magically quit needing as many calories when we hit 50 ... hmmm, another search...)
When I checked on the USDA site, they give this same info but without a high end, since they note no negative issues with exceeding minimum requirements.
Sites associated with diabetes and high cholesterol all aim towards higher ranges (minimum 25-35g for women) with notes that soluble fibre has been proven to help minimize sugar spikes and lower LDL.
So - it looks like both the 21g and the 25g-35g are basically correct, depending on whether you want just the absolute minimum based on an average caloric intake, or if you want a bit more in order to take advantage of the benefits of more fibre.
Thanks again for asking this question - I enjoyed learning more about this!