On p31 and 32 of Roger Wardale's book "In search of Swallows & Amazons" there are some age details which Ransome wrote while planning S&A. They are also given by Christina Hardyment (CFT p22), who also gives the proposed age of Tom Dudgeon as 12 or 13 (CFT p123).
The first version is as follows:-
Amazons: Jane 13, Mary (proper name Ruth) 12, Tom 3.
Then there is a new version:-