How old is Bella? (WAS: Bellatrix as 'Bella)

[Kirstini]

No: HPFGUIDX 69379

The Sergeant Majorette places her at about Lucius' age, and I myself 
had always assumed that she was ages with MWPP, based on 
Sirius' "gang of Slytherins" comment in GoF. She has to be at an age 
where Snape would have associated with her, doesn't she? However, I 
noticed that in "The Noble and Most Ancient House of Black" Sirius 
mentions that he hasn't seen her since he was Harry's age. "Ah," I 
thought. "So that means she was two years older, and left Hogwarts 
at seventeen." The trouble is that that really *would* make her 
Lucius' age, or just one year younger. What I don't understand, in 
this case, is why Lucius himself wasn't included in the "gang of 
Slytherins"? Also, would a third year, for example, associate with a 
first year? Narcissa and Andromeda are also problems. I want to 
order them, age-wise, NAB, but the family tree went the other way 
round. Can anyone do maths?
Kirstini
head throbbing