Depends on your definition of adulthood.
I was raised by my father as a single parent. He grew up in an all-boy family, and had already had a falling out with his mother. For that reason I had to kind of "grow up" a little bit fast.
There were some awkward moments, but we always respected each other's privacy.
Dad's mom was totally anti-enema, at least for boys, and that's all she had. He always, even once he was still living at home after he was 18, had to take enemas completely behind her back, and hide the equipment from her.
She was from a different era, and was never really comfortable with her own sexuality, so she was of no help at all when I needed an enema.
He gave me enemas until I got my first bra. After that, it was self-administered entirely, with both bedroom and bathroom doors open, and Dad at home, so that he could bale me out if anything ever went seriously wrong. Between the time I turned 16, and the time I left for college, the deal was, "I don't need to monitor every enema any more, and you can do them while I'm at work but DO be honest about what you are doing and when. and you can make your own decisions."
Essentially I was not allowed to hide my enema taking from him. He did want to know when I was taking an enema and what I was taking, but I was allowed to decide when to take enemas.
As a result, I don't know that there really was a "First enema as an adult" unless it was the cleanout I did about a week before leaving for Co9llege. That would be the first one I remember after I turned 18.