I think the answer to should cadets training with the firefighters (I think by that you mean at the same training sessions), is "it depends"
If you have an 11 year old join CFS, and train alongside the seniors, it can create issues in terms of the 11 year olds ability to comprehend the information provided, at the speed it is presented.
Some 11 year olds are very switched on, and can (and do) grasp the concepts pretty quickly, so no special attention is needed, while others may struggle to understand the information being provided in an adult learning environment.
Some of the cadets (and some seniors for that matter!) have a short attention span. This can be disruptive, and difficult for the brigade to train the cadets alongside the adults.
If you have a large number of cadets, it can create issues when training alongside the seniors, in that you don't have enough space on the appliance to cart everybody around, not enough equipment to use during training etc.
So there are some certainly some considerations to be taken into account.
However, there are also many advantages of having cadets training alongside the seniors. My own brigade currently has three cadets - 13, 14.5 x2
All three of them were very keen to join CFS, are able to grasp the training concepts presented without having to make extra special arrangements for them.
The attention span on all three is good, so we don't have bad behaviour disrupting the rest of the members.
When they become firefighters, aged 16, they will have had 18 months + of training alongside the Seniors, picking up snippets of info about real calls, so their transition from cadet to firefighter is relatively seemless.
Overall, there are both advantages & disadvantages of having Cadet members training with the seniors.
How a brigade ultimately runs the cadets is dependent on many factors, (some outlined above), and what works well in one brigade, may fail dismally in another!
Pip