My first time with a professional: 18. When I was in my teens living in Kansas City in the mid 1970s, there was an ad in The Advocate that always caught my eye (from a November 1976 issue of The Advocate):
When I had just turned 18, I visited New York for the first time to go to a comic-book convention and had a room in the Con hotel, which was not far from MSG. When I had a free night, I called the escort and arranged for him to come to me. I'm not sure what I was expecting from the ad, but it wasn't that he would show up in a nearly full-length fur coat. He was very attractive, but I think my surprise showed on my face, as he explained he wore it to dress up so hotel security wouldn't hassle him. Still, his appearance threw me, which, combined with my first hiring experience, essentially froze me. I didn't want to hurt his feelings, and went through the motions but just wanted to get the encounter over. He came, but I didn't. I believe the experience was $50 plus $20 tip.
My first time paying? When I was about 16 and paid a neighborhood boy a bit younger than me to allow me to spank him. We kept up that arrangement on and off for a few years, expanding it into other activities as well.