The minimun age for a cadet is 11 years old, so if your 16yo fire firefighters have been a cadet for more than 5 years, your brigade is doing something wrong...
Young members hanging around the brigade with their parents however is a different matter...
I think it comes down to what experience is needed to be a Lt or Captain... In a rural brigade that only gets 10 calls a year, the captain thats been a member for 15 years still might only have the same experience as someone who's been in Dalkieth for 6 months.
If an 18yo has the experience thats needed to be in a position of power, then no one should have a problem with that.