Finding the Holmes siblings approximate birth years, except it's not approximate. And it's all thanks to this scene from His Last Vow.
Officially deceased 2011-2013.
This means the fall was in 2011.
Sherlock is 28 in the first episode, but then we have Christmas in the next episode. Then, it moves on to the next year, and Sherlock is born on January 6th, so his birthday passes and he's 29.
The Great Game happens. And then next series, next episode, another Christmas, another birthday. 30.
Nothing important in The Hound of Baskerville. But The Reichenbach Falls, which we now know happens in the Autumn of 2011. This means it was 2009 that A Study in Pink is set in.
And the deceased thing says to 2013, so that means 2 years pass, not three. 32.
In 'His Last Vow, there's another Christmas. And then after Mary's death, Sherlock's birthday rolls around in The Lying Detective. 33.
It is now 2014. The Final Problem is in 2014. Sherlock is 33, Euros 32, and Mycroft 40.
And that means this year, Mycroft is 45, Sherlock is 38 and Euros is 37 [though, Euros and Mycroft's ages depend on when they are born, I believe Euros to be born in Spring, and Mycroft Autumn. So this would make Mycroft still 44.]
Mycroft, 1974. Sherlock, 1981. And Eurus, 1982.
I hope you enjoyed this. My head literally started hurting after trying to remember if there was a Christmas in A Scandal in Belgravia.