Unhappy under 18's policy is laid down in BR8748, haven't got a copy on me at the moment but off the top of my head no notice is required, go to your DO tell him/her, they will counsel you to ensure you are making the decision for the right reasons and you could be out in as little as a week. Just make sure you are unhappy! the grass isn't always greener on the other side and chances are you have no real insight into the RN yet as you will probably be still in training !
You may submit two weeks notice to leave the Service at any time after you have served 28 days up until you have served 6 months.
Up until the the age of 18, you may be discharged (SHORE) "Unhappy under 18" - not called Junior anymore, at Commanding Officers discretion. (BR8748, Change 18, Chapter 10, Art 1010, para 6). Off the top of my head (NOT!!!)
All others: After this period, you must complete your Standard Initial Training Period (SITP) & then complete 2 years 6 months return of service before you can submit 12 months notice to leave.
It is important to note that your SITP varies from Trade to trade, as indicated in BR 8748, when they finally manage to keep track of all the changes in training! (eg: Raleigh extending to 9 weeks phase one, branch name changes etc.,)
The new contracts are either 18 years long or up until the age of 40 years, whichever is longer.
(Artificers & Nursies, have to complete 3 years, 6 months return of service after completing SITP)
In answer to the original question:
Is it correct that a Junior Rate under the age of 18 can still leave at anytime before there 18th Birthday with very short notice ?