Some of them are 8-10 years old. Grade 2-3.
The band is for experience of ensemble playing and listening to band mates. Not academic study. They get that with their individual teachers.
If the youngest ones walked in and saw new key sigs they have never seen in their individual lessons they'd walk straight out again. We find a way to get them playing, even if we have to give them the 4th part and tell them to only play the first note of the bar until they get comfortable with trying the 2nd note.
They join the ensemble as nervous wrecks and within 3 months the confidence is already going up and they start asking for more fun / difficult things to play.
The 16-18 year olds are more than capable of playing in any key but they are there for fun, having been there for 10 years themselves. They tend to be in county level bands as well, some of them aiming for a conservatoire place.
It's a fantastic band and I'm constantly amazed I get to play with them. I got press-ganged when the actual bass teacher quit and moved away. I was given the book and 30 mins notice for my first rehearsal with them.