in most places dentists do not refer until the child is ready as if you refer early the waiting list clock starts ticking, and if they are not ready it just wastes a lot of the orthodontists time seeing someone and saying you are not ready come back in 6/9/12/18 months (i'm a dentist not orthodontist and inappropiate / early referrals can mean trouble as your referrals then tend not to be taken at face value) there will be local guidelines to follow which may well depend on orthodontist availability
there is a big shortage of orthodontists ( this is not really about plenty but they are private there is a shortage full stop) hence strict criteria for NHS orthodontics , it is the only speciality in NHS in which I believe waiting times are not penalised, the 13/18 week from referral to first appointment does apply so it is irresponsible of a dentist to refer early as they know it is clogging appointment books which are already overloaded.
on a personal level for a patient and their parents they would like the referral and being told wait 12 months as it is kind of reassuring, instead of not being referred. also early referrals can result in less than optimal treatment, most orthodontics is best done to co-incide with teenage growth spurt, they are some conditions which do benefit from early treatment while there are still some baby teeth, but most is best done when last baby tooth has gone and all permanent teeth are erupted except wisdom teeth
However there are no absolute rules, so you can't ever say without seeing patient oh it can be done without extractions, headgear or functional appliances
maddie some problems that involve the skelton of face can never be corrected at any age by braces alone and surgery will be needed , fortunately this is a very small number and when it is needed it is done after all growth finsihed which is often aged 18 plus
for information only the average cost of a course of treatment is £3000 and takes 18-24 months with appointments roughly every 6 weeks, but that is an average there is considerable variation