How old are Sherlock and Mycroft Holmes in the BBC Sherlock TV-Series? Do they have other brothers and/or sisters?
For there is no reference in the show, one can only assume that they are the same age as the actors:
- Sherlock: 34 in S1 (Benedict Cumberbatch, born in July 19, 1976)
- Mycroft: 44 in S1 (Mark Gatiss, born in October 17, 1966)
If you are going with the canon, however:
- Sherlock: 27 in S1 (In His Last Bow, which is set in 1914 Sherlock is described as "He was a tall, gaunt man of sixty, with clear-cut features" so he must be born around 1854 and he first met with Watson 1881)
- Mycroft: 34 in S1 (In The Greek Interpreter Holmes tells Watson that Mycroft is "Seven years [his] senior")
As pointed out before the show makes a reference to "another one". So there is a possibilty that there is another Holmes brother. But at the end of S3, we can only guess.
According to the novels A.C.D. wrote, there is no other brother mentioned. But a possible explanation that there might be one, is given here:
Holmes once stated that his family were country squires, which means that the eldest brother would have to stay to manage the house. If Mycroft were the eldest then he couldn't play the role he does in four stories of the Sherlock Holmes canon, so Sherrinford frees them both. This position is strengthened by the fact that Mycroft's general position as a senior civil servant was a common choice among the younger sons of the gentry.
Since the series takes place in the 21th century, there is no need for the eldest brother to manage the house. This only leads us back to guessing ;)
Here is an answer with spoilers from the Series 4 episode The Final Problem (transcript).
JOHN: So there were three Holmes kids. What was the age gap?
MYCROFT: Seven years between myself and Sherlock, one year between Sherlock and Eurus.
JOHN: Middle child. Explains a lot.
So, in summary (more spoilers):
Mycroft is eldest by seven years, then Sherlock, then a sister Eurus - whose life is a dark family secret kept by Mycroft - a year younger than Sherlock. The actual age of any of the siblings is never stated.
Though there has never been a direct mention about Sherlock's or Mycroft's age, but in the episode The Empty Hearse, there is a slight reference.
SHERLOCK: I didn’t know you spoke Serbian.
MYCROFT: I didn’t, but the language has a Slavic root, frequent Turkish and German loan words. (He shrugs.) Took me a couple of hours.
SHERLOCK: Hmm – you’re slipping.
MYCROFT (smiling tightly): Middle age, brother mine. Comes to us all.
Regarding the third Holmes sibling, nothing has been made clear yet. Hope the coming seasons will reveal the mystery.
Source (Transcript) : Ariane DeVere