I think it depends on the maturity of the individuals and the circumstances. FWB really is not anything new. Many (seniors) have been doing this for decades. Maybe they're a widow/widower or divorced and have NO interest in a full-time relationship, living with someone, nor have a desire to re-marry. A "friend" may come into town for the weekend or they go out of town to visit a "friend".
More often than not this is someone they do not bring around their family and platonic friends very often. They keep their private life (private). Their own adult children and their grandchildren may not even be aware that they are "getting their grove on" from time to time with a "special friend". As far as they know "gramps" is living a lonely life. :-)
Having said that a FWB arrangement for someone who is young or immature may not be ideal. Subconsciously they may want more than a FWB but are settling for it now in hopes of it evolving into more. There is also the chance that one of them becomes "emotionally invested" and insists on becoming exclusive. Either way someone gets hurt in the end.
Bottom line is (both) people have to truly know themselves and each other. This is something that seniors pretty much have down.