I have younger sisters and I am sure I had seen them at some point when we were very young. I honestly can't remember any occasion that I did though.
So for me, I have to say it was on my 23rd birthday.
I was dating a very beautiful brunette girl. I was always amazed that this beautiful young woman wanted to be with me.
We had done lots of heavy petting through out our dating. I had slipped my hand into her panties and fingered her wet pussy many times. She, in turn, had pulled out my cock and stroked me off or sucked me many times.
We had the house to ourselves and she asked me what I wanted for my birthday. I said I wanted her in my bed.
We were in my bed, making out as we removed different articles of clothing.
I finally got to her jeans and pulled those off, then her panties. I loved seeing her bush and I wanted to delve into her folds.
I lay between her sweet thighs and began licking, nibbling, and kissing her beautiful pussy. I was slowly working my tongue inside her and working on her clit. (I had read about the female clit in Penthouse and Playboy) It was delicious!
That birthday gift was then interrupted by my door bell ringing. Some of my family had come home very unexpectedly and ended our play time.
I loved that birthday gift and wanted that beautiful brunette to be mine. I eventually married her and I get that beautiful woman on every birthday now.