Link to short story that JKR wrote for charity is now on Leaky

[Carol]

Alla wrote <snip>:
> 
>  I know what can be wrong with arrogance in general. And, I will not
surprised at all if at seventeen or eighteen James and Sirius indeed
already had quite a lot of that experience that caused them to be
arrogant or confident, depending how one looks at it.

Carol responds:
I'm not going to comment on their arrogance except to say that they
have a lot to learn and seem unchanged from the "arrogant little
berks" we saw risking the lives of the people of Hogsmeade in their
fifth year. I wouldn't call it confidence, more like insolence and the
belief in their own invincibility, but, oh, well. You like them and I
can't stand the little berks. :-)

However, with regard to age, James is certainly eighteen if he's out
of Hogwarts and a member of the Order of the Phoenix given his March
birthday. We're not given Sirius's birthdate, but chances are good
(five out of six, reduced to lowest terms from ten out of twelve
months) that he also turned eighteen before leaving Hogwarts. Snape
refers to him as being sixteen when he pulled the so-called Prank near
the end of their fifth year and JKR says on her website somewhere that
he was "about twenty-two" when James and Lily were killed, whereas
they were twenty-one. If "twenty-two" is correct, he must have had a
September or October birthday, making him one of the oldest students
in his year (like Hermione and Angelina in their respective years).  

At any rate, I think that eighteen is more than seventeen for both of
them.

Carol, who agrees with Alla that seventeen through nineteen qualifies
as "late teens" (JKR used that description for Barty Crouch Jr. at his
sentencing when Barty was about nineteen)