Viagra doesn't actually make anyone horny, anyway. It just facilitates the blood flow to the genitals. Old guys take it because the thing about male libido going down as you age is a myth, they're still as ragingly horny at 70 as they are at 17, they just can't do anything about it without medicinal intervention (´ー`)

Interestingly if a woman takes cialis, it works in a similar way. The increased blood flow goes to the clit (´ー`)