The optimal circumstances for FWB may be those of young people before marriage and older people who have grown children.

I agree. For older people with grown children, what often happens is that a couple who might seem to have been sexually matched in younger years find themselves seriously mismatched in their later years, yet may otherwise have a relationship they value and want to preserve for many reasons. For couples who are reasonably open-minded about this situation, it makes little sense to live by society's judgmental opinion that your erotic life always has to align with the same person with whom you jointly file your federal taxes.