They're thinking about something: I pitched a fissy hit in a post yesterday about, not Sherlock's age, but his boyness (I'm deliberately using that word instead of immaturity). In canon Mycroft is 7 years older than Sherlock, in BBC the apparent difference looks like 15, but could be as much as 20 years. In canon Mycoft appears twice. Unovis commented on my LJ that, in effect, by expanding Mycroft's role they've split the character - with Gatiss embodying the insider/conservative aspect of the character and Cumberbatch the outsider/queer (for a certain value of queer) aspect - it's not a perfect parallel, since we see Sherlock celebrating Christmas with friends and Mycroft alone but... Unovis commented further that exploring the split would be delicious; I'm pondering that.