Okay I have to talk about Arthur's revealed birth year (1895) now because it makes the Shelbys ages sooo fucked up.
NOTE: Edited 4/28 with some corrections and additions.
I previously estimated the Shelby siblings birth years in season 1 (1919) as part of a conversation about whether Ada or John is older. Repeating some of my observations and adding some:
Finn is stated to be 10 years old [edit: but about to turn 11] in the first episode of the show in 1919, so I infer his birth year is 1908.
Freddie and Ada are ~3 years apart ("I've loved her since she was nine and I was 12"). Freddie and Tommy were friends since school so probably are around the same age (therefore assuming Tommy and Ada are also 2-3 years apart). I get the impression that Freddie is slightly older than Tommy from the way he talks to him, so I'm going with a 2 year gap between Tommy and Ada (also because of the story about them carrying a bucket together).
John tells Polly in season 2 that by the time he was Michael's age (which was 17) he had watched hundreds of men die. Meaning John was underage for a fact when he enlisted with Arthur and Tommy. (Note: None of the Shelby siblings have birth certificates, and people as young as 14 lied and served in WWI).
A revision and addition: In season 2, Tommy writes to Churchill talking about two battles he was at that took place in 1916. [Note: I previously said Tommy and Greta took a pre-war-Tommy photo in 1915 but I misread and it was actually 1913. This makes it more ambiguous when the brothers enlisted, but at least by 1916]. [edit: In the wine cellar in The Immortal Man, Tommy's narration says he and Arthur enlisted in 1914]
Another addition: I was reminded by littlepeakydevil that we also know the age gap between Arthur and Tommy. It's three years (this is revealed in 6x04 when Tommy tells a story about them fighting).
NOW we have a birth year for Arthur which puts an upper limit on everyone else's age it's pretty wild.
We are told in Immortal Man that Arthur was born in 1895.
Finn's birth year is 1908.
I will assume Tommy (since he's 3 years younger than Arthur) was born 1898. We could fudge this a bit and say Tommy was going to be 10 the year he had that fight with Arthur, so we could make him an 1897 baby.
Ada and John are even more ambiguous, but I suggest: Ada is a 1900 baby and John is a 1901 baby. You can swap their birth order too, but again, partly because of the bucket story, I think Ada is next to Tommy in birth order. If we make Tommy an 1897 baby, we can also push Ada and John to 1899 and 1900, respectively, but that's as close as we can really put everyone in age.
Using primary suggested birth dates for Tommy (1898), Ada 1900), John (1901), and Finn (1908):
In season 1 (1919): Arthur (1895 baby) is 24. Tommy (1898 baby) is 21. Ada (1900 baby) is 19. John (1901 baby) is 18. Finn (1908 baby) is 11.
In season 2 (1921-1922): Arthur is 26-27. Tommy is 23-24. Ada is 21-22. John is 20-21. Finn is 13-14.
In season 3 (1924): Arthur is 29, Tommy is 26, Ada is 24, John is 23, Finn is 16.
In season 4 (1925-1926): Arthur is 30-31, Tommy is 27-28, Ada is 25-26, John is 24-25, and Finn is 17-18.
In season 5 (1929): Arthur is 34, Tommy is 31, Ada is 29, Finn is 20.
In season 6 (1933-1934): Arthur is 38-39, Tommy is 35-36, Ada is 33-34, Finn is 25-26.
Movie (1940): Tommy is 42, Ada is 40, Finn (if still alive? maybe not?) is 32.
If we go with my closer packed together dates to bring John's age in 1919 up a bit (because he has FOUR KIDS and though it did happen, 15 would be SUPER young to enlist, and Arthur does suggest John joined at the same time as him and Tommy in 4x06) then we have Tommy (1897), Ada 1899), John (1900)âFinn and Arthur's ages don't move.
In season 1 (1919): Arthur is 24. Tommy is 22. Ada is 20. John is 19. Finn is 11.
In season 2 (1921-1922): Arthur is 26-27. Tommy is 24-25. Ada is 22-23. John is 21-22. Finn is 13-14.
In season 3 (1924): Arthur is 29, Tommy is 27, Ada is 25, John is 24, Finn is 16.
In season 4 (1925-1926): Arthur is 30-31, Tommy is 28-29, Ada is 26-27, John is 25-26, and Finn is 17-18.
In season 5 (1929): Arthur is 34, Tommy is 32, Ada is 30, Finn is 21.
In season 6 (1933-1934): Arthur is 38-39, Tommy is 36-37, Ada is 34-35, Finn is 25-26.
Movie (1940): Tommy is 43, Ada is 41, Finn is 32.
Thanks for coming to my TED talk.