Boy are you confused.
Any older woman's desire to have sex is no different than an older man's. The more fit and the healthier, the more sex. You can't expect old people with arthritis and rheumatism to want a roll in the sack every day. OTOH, those who respect and take care of their bodies will never fear sex.
Your take on sex by age groupings is also interesting. But my question is, if you have an attractive middle-aged gf of 10 years with a small son, why have you not asked her to marry?