Any search for a UK casualty in WW1 and WW2 should always start with the Commonwealth War Graves Commission website as it should cover all such deaths. This may, of course, be the site you found! There is only one William John Miller in the Naval casualties during WW2 - the one you found. However - a word of caution - bureaucracy is never as careful as we'd like it to be and it's possible that the Royal Navy might have known him as just William Miller. In which case there are 3 William Millers to worry about, a couple of whom have ages and parents, so hopefully you can knock them out of the reckoning.
So now I reckon you need to decide which of the Williams or William John is yours... The unfortunate thing is that the WW2 service records are still held by the Royal Navy and it will take some time (and money) to get copies. Try and get your head round the different aspects by looking at the guides for Royal Navy personnel from The National Archives. I'd take a guess that you'll need to read up on ratings from the Royal Navy and the RNVR for the post-1920s era.
I suspect that once you've got your head round these simple guides (yes - sorry - these are the simple ones - it's real life that's complicated!) then it might be useful to get yourself to a decent library and read bits out of 1 or 2 books - "Tracing Your Naval Ancestors" by Simon Fowler is the latest, I think.
It just occurs to me that there are death certificates for WW2 casualties in what's called the "GRO War Death Naval Ratings Indices (1939 to 1948)". The index for these is a/v on FindMyPast, so you could use this to find the reference for the HMS Blean death, send for the certificate and it should tell you the age. Well, it should if it's like a normal certificate, but I've never sent for one so I can't confirm.