When I was around 14 or 15 I was friendly with a guy who lived nearby. His mom was, at that time, I imagine in her early 50's but she had a great body and fabulous tits.
She was a bit of a 'prick-teaser' in that she would often lift up her skirt and say things like 'look at my new panties'.
She always wore dark tan stockings and the sight of her white thighs above the tops and her panties were the things I fantasized about most nights when I masturbated.
One day I went to my friends house, but he wasn't back from work.His mom, we shall call her Grace, was there and she told be to sit and wait for him. I sat in a chair slightly to the side of her and the way she was sat, I caught a glimpse up her skirt and could see her stocking tops and a flash of her white panties.
I was instantly aroused and my cock was rock hard. I deperately wanted to masturbate, but couldn't without being very overt and I dared not do that.
However shortly afterwards, Grace appeared to go to sleep. She drew her legs up and this enabled me to have an unobstructed view up her skirt, of her legs clad in dark tan stockings. I could see her stocking tops and her white thighs and I could also see her white panties stretched over her pussy.
I just had to wank and gently took out my cock under the cover of my jumper and after a few strokes shot my cum which landed on the seat and on the floor.
It was the most intense orgasm that I had ever had. Grace didn't move or give any indication that she'd seen what I had done, but from that day on, everytime I visited my friend's house and she was in, she always gave me an 'accidental' view up her skirt.
Grace remained the subject of my masturbatory fantasies for many years and even now I still masturbate thinking about the times I saw up her skirt.