Even though it's actually taken this long for me to start doing this, I now stroke the bottom of my balls and occasionally my perineum while masturbating.
I'm almost 27 and, what's more, I spent two years telling people something like, "when you stroke something along the length of your perineum, you may experience a brief pleasant sensation," and it only occurred to me that I should try doing so about three days ago. Annoyingly, although the rest of me is hypersensitive, my penis isn't actually that sensitive, so although this gives me a lot of staying power, sometimes it requires a bit of extra sensation to get me off (ref: extra stimulants; sub-ref: Catharine sucking my right nipple; soft porn; stroking my hair). Well, cupping the underside of my balls is one such stimulant.
It's only me as well. I've stimulated other people's areas for sexual pleasure as well. Nobody with balls, evidently; I hold true, though, that the area of skin between vagina and anus is oft overlooked in the practice of oral sex and should not be avoided. But nevertheless, I forgot about trying it myself.
A few days ago, my left hand slipped and accidentally brushed against my perineal raphe, which made me go audibly "ooh!". While gripping my penis' foreskin with my right hand, I brushed my index finger of the left underneath my scrotum and along the line. It sent a tingling sensation throughout my sexual organs, producing a sensation not unlike the slight tickle that I feel in my balls when I am suitably interested enough to get aroused. And, due to the fact that the testes tighten up during sexual arousal coupled with the fact that I have freakishly large hands, it wasn't any trouble to continue cupping my sack and occasionally stroke myself there while continuing to tug on the penis.
Since this happened - you know, by accident - I've continued to do it when I masturbate, just to bring that extra edge of pleasure to my playtime. And it only took me 27 years to realise this. Fantastic.