Thanks, Admin, for bumping this topic. It is one of my favorite interests.
For me, the short answer to that question is many decades, essentially all of my life. It seems to me that I was born with a strong interest in female peeing, starting quite literally at my mother's knee.
The long answer is a bit of a story, and I apologize in advance for the length of it. But I think it contains some interesting insights and history.
I was born and raised in the Midwestern US, during World War 2. At that time, most women wore dresses or skirts, rather than pants, all the time. Underneath them, they wore corsets or girdles, with garter tabs (suspenders) to hold up their rayon or nylon stockings. Some of them just wore garter belts, for the same purpose. Panties (knickers) were worn over their corsets, garter belts, or girdles to make it easy and quick for them to slip them down to pee. A woman could simply lift her skirt and slip her panties down, in one motion, as she sat on the toilet, gathering her skirt over her thighs, to block the view of others. She didn't have to undress any further than that to pee. She might show a flash of thigh, above her stocking tops, and below her other clothing, but that was the limit of exposure that she risked. Under those circumstances, it was fairly common, at least in my family, for women not to bother shutting the bathroom door, when they peed. I found it amusing to hear my mother and one of my aunts complaining about my grandfather having built his house so that he had a direct view of the toilet, through the open bathroom door, from his easy chair!
As a toddler, at two or three years old, I followed my mother into the bathroom whenever she peed. I was an only child at that time, and my father was away all day, working. She almost never closed the bathroom door, at home. Since she did it with her legs together, and her dress or skirt covering her upper thighs, I couldn't see anything interesting, but I could hear her tinkle, and rarely hiss, and I could smell her pee, which I liked, for some unknown reason. I tried very hard to look between her smooth bare thighs, above her stocking tops, or under her thighs, from the side, but she was very skilled at hiding her stream. She always wiped from the rear, with her legs still together, so I couldn't see anything there, either.
As a toddler, I was very interested in what was under women's skirts, and I think my interest in their peeing was just an extension of that interest. I was intensely interested in how and where their pee came out, and I asked my mother once why she had to wipe, when I, as a boy, didn't have to. She told me it was because ladies got wet when they "wee-wee'd" (her term), and told me that I shouldn't ask such questions, because it wasn't "nice." This just added to the mystery of female peeing, in my mind, and stimulated my curiosity.
When I was about four or five years old, my mother would bring me with her when we were out shopping, and she had to use public ladies' rooms.There was not enough room in the toilet cubicles for me to stay in one with my mother, while she peed. I loved that, because, at my height, I could walk around and look under the stall doors at the other women peeing. Some of them did it with their legs apart, which was exciting, but I usually couldn't see very much, because their underwear usually blocked my view, at their knees. I did see a stream or two, between some women's legs, through the gap in the front of the public toilet seats, if their panties were below their knees, and their legs were open. For me, it was a revelation and a thrill just to see a woman pee with her legs apart, since my mother always did it with her thighs pressed tightly together, blocking any chance for me to see her stream.
Interestingly, although the public toilets weren't particularly clean, in that time, none of the women I was able to watch squatted or hovered above them. That would have given me a clear view of everything, satisfied some of my curiosities, and a huge thrill. But at that age, I had no idea that women could squat when they peed, or might prefer to do it that way, rather than covering the seat with toilet paper, as my mother usually did. I thought that they had to sit all the way down, like my mother. When my mother found out what I had been doing, while she was busy peeing, she told me that was really not very nice, and made me wait for her outside of the ladies' rooms, which was a big disappointment to me. My interest at that time arose purely out of curiosity about females and their secret bodily functions. I was too young to make any connection to sexuality. That came a little later.
When I was in elementary school, I think it was first or second grade, there was a little girl in my class, who always smelled a little strange, vaguely like slightly stale pee. I actually liked her smell, but other kids in the class sometimes talked about it behind her back. She was the smallest, shortest girl in the class, with curly hair, like "Little Orphan Annie," and she wore the shortest skirts, so it was easy for bad boys like me to look up her skirts.
All the kids, boys and girls, usually peed when they came back in from recess, and I was very lucky, one day. I was already sitting at my desk, when she came in from the little girls' room, and slid into hers, briefly spreading her legs as she got into her desk, giving me a perfect view of her inner thighs and white cotton panties. What I saw totally amazed me. I saw a small yellow spot in her white panties, which looked like it was still wet, and I could definitely see several drops of yellow liquid, it had to be pee, clinging to her bare inner thighs. I was intensely curious, wondering how her pee came out, and how her legs got wet. I had never seen a girl my age pee on a toilet, at that time. I knew that they sat, because my mother and other women sat, but I couldn't see how her pee came out, or what parts of her got wet. My younger sister was a baby, still in diapers at that time, so she was no help.
I was so intensely curious about what I had seen at school that day, that I finally summoned the courage to risk asking my mother, at home. I said, "Mom, I saw a girl with "wee-wee" (my mother's term) on her legs, at school today. How did that happen?" Instead of answering my question, my mother asked me how I saw what she had on her legs? Did I look up her skirt? That was not nice, her wet legs were none of my business, and I should be ashamed of myself for doing that. I admitted that I had looked up her skirt, and her panties had a wet spot, too. But I still wanted to know how her legs got wet. My mother finally relented, and answered my question, saying that her mother probably didn't teach her how to sit on the toilet properly, so her legs got wet, and her mother obviously didn't teach her how to wipe herself properly, so she didn't get her legs dry, and her panties got wet for the same reason. My mother said that must have been very embarrassing for the little girl, and I shouldn't have been looking at her. I said she didn't seem to know or care if boys looked up her skirt, so I didn't think that she was embarrassed. Besides, she usually smelled like "wee-wee," and the other kids talked about her, but she didn't seem to know or care.
My mother's answers didn't satisfy me at all, but that was as much as I was going to get from her. From that time on, I had an intense, secret curiosity about how girls' pee came out, how they sat when they peed, and whether their legs got wet, which brought up the vivid, exciting memory I had, of that girl's wet thighs. My curiosity continued to grow, from that time on. In spite of what my mother had told me, I wondered if all girls' and women's legs got wet when they peed, and most of them just wiped the dribbles away. If they did it with their legs together, as my mother did, how could they avoid getting them wet, if their pee came out frontward, as I believed it did? On the other hand, I had never, ever seen my mother wipe her legs. She always wiped her rear, with her legs together. So I was totally confused, and more curious than ever. This curiosity got me completely hooked on female peeing, for the rest of my life.
My curiosity intensified, as I approached my pre-teens, and became aware of sexuality. I immediately associated sex with peeing, before I even knew what sex was. At one point, I thought that sex involved girls and boys getting their organs together and peeing on each other, in some way. I didn't get to try that until a lot later in life!
But the final, straight answer for me, to the question posed in this forum is 74 years ± 1 year.