I think I started going to interview customers with my boss very early on my first job in the 90s. I've been given a team to manage after two years and I started doing interviews, analysis, requirements, etc on my own. Basically I was a senior engineer by then. The company was always very supportive so if I had to ask for advice I had somebody to ask to. After all 1/n-th of my boss' career depended by the outcome of my job.

Of course I'm much better at that kind of job now. I wonder how I could really understand my customers but obviously it was good enough.