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:-
- Swallows: Dick (scored out) John 12, Susan 10, Titty 8, Roger 6.
- Amazons: Jane 13, Mary (proper name Ruth) 12, Tom 3.
Then there is a new version:-
- Swallows: John 12, Susan 10, Titty 9, Roger 7, Victoria (Bridget) 1 1/2.
- Amazons: Jane (scored out) Nancy 13, Mary (scored out) Peggy 12, Tom 3.
Both those early versions were later superceded.
In Arthur Ransome - Master Storyteller, Roger Wardale gives the most definitive ages:
- "For those who would like to know exactly how old Ransome thought the Swallows and Amazons were, there is a single sheet of paper written about this time  that clears the question for good and all. In Swallows and Amazons (that is placed in 1930 — not 1929) John and Nancy are 12, Susan and Peggy are 11, Titty is 9 and Roger is 7. By the time of the proposed sequel to The Picts and The Martyrs in 1933 that he called ‘Swallows & Co’, John and Nancy are 15, Peggy and Susan are 14, Dorothea is 13, Titty, Dick and Joe are 12, Bill is 11 and Roger and Pete are 10."
This also agrees with the chronology published by Arthur Ransome in The Best of Childhood.
It seems that Arthur Ransome finally decided on 1930 as the year of the first book. See 1929-1930-1931 paradox.
We can therefore give Ransome's chronology and the ages of the main characters by book, putting together the six books set in 1932 in one group, although the ages will vary by about 8 months over this period. This seems reasonable, as the ages are approximate anyway.
Swallows and Amazons, Summer 1930
- John 12, Susan 11, Titty 9, Roger 7
- Nancy 12, Peggy 11
Swallowdale, Summer 1931
- John 13, Susan 12, Titty 10, Roger 8
- Nancy 13, Peggy 12
Winter Holiday, Jan-Feb 1932
Coot Club, Easter 1932
Pigeon Post, Summer holidays 1932 - first two weeks
We Didn't Mean to Go to Sea, Summer holidays 1932 - fourth week
Secret Water, Summer holidays 1932 - last 9 days
The Big Six, Summer holidays 1932 - last two weeks
- John 14, Susan 13, Titty 11, Roger 9
- Nancy 14, Peggy 13
- Dorothea 12, Dick 11
- Tom 14?, Twins 12-13?
- Joe 11, Bill 10, Pete 9
The Picts and the Martyrs, Summer 1933
- Nancy 15, Peggy 14
- Dorothea 13, Dick 12
Picts in the North, Summer 1933
- Joe 12, Bill 11, Pete 10
Great Northern?, Summer 1934
- John 16, Susan 15, Titty 13, Roger 11
- Nancy 16, Peggy 15
- Dorothea 14, Dick 13