Note the bottom. Birth year is 1977. In this font, I can’t think of any other digit that would partially look like this, is repeated and gives credible birth year. This, for me, settles the age debate.
Interestingly (probably just a prop issue?) the Fall date is absent. I mean, John would notice something like that…
So this means that Sherlock was 33 when he and John first met. Also, that would make Mycroft about 40 at this time.
He’s approximately three years younger than John.
He was 12 when Carl Powers was murdered and he “made a fuss” to the police.
He was about 28 when he first met Lestrade.
He is now 36 years old.
Excellent!! Though, assuming they met on Jan 29th, 2011, Sherlock would be 34. From ASiB, we know John was 37 that year.
So, the age difference is, as you said, 3 years. Which puts John’s birth year at 1974. But above news is from the time after the events at pool. So, it’s possible that John’s birth date could be in 1973 (roughly) July-Dec and he would still be 37 at the time of above news.
Mycroft’s birth year could be 1969-1970. As in, he could have been born anywhere between roughly September 1969 - April 1970 to be called 7 years older. (Stretching it more causes my mind to adjust 7 years to 7.5 years, and that is not a canonical number for me!)
He met Lestrade around the age of 28, meaning he wasn’t just out of University. Actually we don’t even know whether he completed any degree. What was he doing then? Rather, what could he be doing before meeting Lestrade? There is his addiction. Did he kick it after meeting Lestrade? Before? Because 28 years is a long time, most people are well set into their jobs (Dull), and Sherlock invented his after this? Oooh… will there be some inklings to this in series 3 given that there’s going to be some family time (speculated)?
FINALLY! I have always believed Sherlock’s age was similar to Benedict’s. Great sleuthing, cosmoglaut! I salute you.