Calculating Sirius Black' age and birthday
(to read along with Snape's birthday calculations)
It suggests that Sirius got 17 while at Hogwarts
because he sounds as though he got his
place right away when he needed a roof.
That would mean when he was out of school.
However, since Sirius ran away from home at 16, he would have had to run away during summer or Xmas after starting his 6th Year. Running away for a short holiday or during his schooling would be avoidance rather than running away .
No, he must have ran away during the Xmas holidays of his 6th Year or the summer after his 6th Year. However, summer is more probable because at Xmas, he could have chosen to remain at Hogwarts.
Lupin and Sirius are said to be the same age, so this suggests that they were not so far apart on the calendar. Lupin was born March 10th. So if Sirius is the same age, I doubt he was born on Sept 1st which would make him 6 months older and always ahead of his friends. Another proof that his birthday must be later in the school year. Especially since James seems the youngest (see calculations at Snape's birthday).
Hogwarts students who started in 1970 were born in-between Sept.2nd
1958 and Sept. 1st 1959.
Also, Rowling said he'd be about 22 on Nov. 1st 1981. Not nearly 23, about 22. Or else she would have said "about 21-22" or "about 22 -23". Just like she did when she mentioned how old Snape was. I also believe Rowling said that because she wanted to say: "Yes, that's horrible being put in prison at such a young age and the Potters being killed while so young" I think she wanted to say 22 because it represented the age of most of her characters at the time.
From my other calculations, Snape was 22 or 21 and 9months on Nov. 1st 1981. So if Sirius' birthday was in Sept, Oct, Nov or Dec, (near the Fall of Voldemort) he would almost be 22-21 or just had turned 22-21. But Rowling didn't mention 21. Again, it is best to narrow our scope and situate his birthday near Lupin's (January, February, March, April, May).
Afterwards, he went back to Hogwarts for his 7th year after living with the Potters over the summer or part of it.
Therefore, if Sirius indeed follows this pattern and was indeed 22 when he was put into prison (surely a bit before his 23rd birthday), it means that Lupin has the same age and so does Severus. Only James is younger having his birthday possibly from late June and up to August 31st. It is obvious that they were all born (Snape, Sirius, Lupin and James) in 1959 in order to be 22 by Nov. 1st 1981.