To clarify on standards: Diver Age Restrictions, p23 General Standards And Procedures, Key Standards can be summarised as follows:
10 - 11 year olds - maximum depth for training is 12M, after certification, must dive with a certified parent, guardian or PADI Pro to a maximum of 12 metres
For 12 - 14 year olds, maximum depth for training is 18M and after certification they must dive with a certified adult.
PADI pros are required to honour the 10 - 11 year old diving depth restrictions when supervising junior divers but we are NOT required to stick to a hard and fast limit of 18metres when diving with any other open water certified diver, aged 12 and over.
All the forms required to be signed by student and parent/guardian and instructor alike require that these limits be adhered to when beginning training for a 10-11 year old but one could suggest there is a legal grey area because who makes these rules? PADI is clearly not a law-maker, but I wouldn't want to fight the issue if these standards were broken and something went wrong.
For the maximum depths - some dive centres stick religiously to the "adult" training limits, for liability or local legislative purposes. Others do not. A diver is certified to dive in conditions in which they are trained, yes, but the conditions at 20 metres might be very similar indeed to the conditions at 18 metres when diving in the same location - but transferring a tropical resort course OW certification to the Pacific North West means that a PADI shop over there has the right to insist on further training before even getting in the water.
But - the decisions to break those training limits are, technically, the responsibility of the diver, not whomever they are diving with on the day.
As for the original post, the Junior OW depth limitations are often mis-interpreted and I've heard plenty of people say that they thought the restrictions apply to all Junior divers, not just 10-11 year olds. I would suggest that this is because (a ) they took somebody else's advice and (b ) Thought it was perfectly sensible advice and therefore didn't question it.
There are a lot of standards, and it's impossible to remember them all when you don't teach something regularly. For those who think it's disturbing that an instructor can't remember all the standards - consider that in this case, the instructor was wrong, but if they had enforced restrictions on all junior divers to a maximum depth limit of 12m they would be doing something with just that little bit of extra risk removed.
Good grief! Just think - a PADI instructor doing something too safe! What is the world coming to!?
Cheers
C.