I'd like to know what you mean, too, YMB.
Rapists don't rape because they are overcome with lust at the sight of a beautiful woman in skimpy clothing. They rape for power, to shame and degrade their victim, to see fear, to control.
Some rapists believe they can overcome and assault a healthy, strong woman. Others don't have that confidence and choose the weak and vulnerable - perhaps a young woman who has had far too much to drink, a slightly built teen in the wrong place at the wrong time, a frail old lady.
Don't forget the men that are raped.
All rape is wrong and evil and each victim is individual and reacts differently, regardless of age. I think society is more horrified at the rape of an elderly woman because society thinks there is something very wrong with younger men being sexually attracted to older women, such as the relationship that was in the news very recently. Many younger people are repulsed at the very idea of older people having sex.
There is no reason why a 90 year old victim would suffer more than a much younger victim but society is more repulsed by it. Even in the OP the victim is a 'poor 90 year old'.
I hope the offender is caught very quickly and this lady gets all the support she needs.