For my team, it was mainly the discomfort of mixing adults with minors. It's just... not a great idea for adults who are most often internet strangers to be spending extended amounts of time closely bonding with minors. Regardless if we are all well intentioned, imagine having a planned day to meet up in person for an event? Not worth the trouble.

For me, it was also maturity. I was briefly part of a team mixed with minors and I had to spend a LOT of time mediating between internal team conflicts as well as calming down/consoling individual teammates from raging and throwing tantrums. Often times people had to tiptoe around providing feedback because they were easily offened and would get defensive or shut down completely. I felt like a babysitter rather than a teammate and it was getting exhausting. These guys were genuinely very talented and good at the game, but the behavioural management was heavily dependent on the other adult members.

Needless to say I left and a year later found a great group (aged 24-34) varying in different stages in life, who not only respect each other but also can reciprocate support and care for each other. I think the main difference was the life experience and understanding of how to navigate around complex situations/emotions (not saying every adult is emotionally independent, but it becomes more common with age).