Rosier, Rookwood, Snape's gang of Slytherins

[ellecain]

No: HPFGUIDX 144562

While rereading HBP, I was struck by the a couple of sentances 
uttered by Dumbledore that confused me thoroughly as to Rosier's age.

I got canon here(for once) - DD says 
"Then if I were to go to the Hog's Head tonight, I would not find a 
group of them - Nott, Rosier, Mulciber Dolohov -awaiting your return?
Devoted friends indeed, to travel this far with you on a snowy 
night, merely to wish you luck as you attempted to secure a teaching 
post"
HBP - Lord Voldemort's request - pg 416 UK edition

So from this one would infer that Rosier is at least as old as 
Voldemort, only maybe a couple of years difference, anyway, since 
they are referred to as "friends".
But I always though Rosier was a "friend" of Snape's not Voldemort's 
because of this piece of canon here:

Sirius held up his fingers and began ticking off names.
"Rosier and Wilkes - they were killed by Aurors the year before 
Voldemort fell. The Lestranges - theyre a married couple - theyre in 
Azkaban. Avery - from what I've heard he wormed his way out of 
trouble by saying hed been acting under the Imperius curse"
GoF Padfoot Returns pg 461 Aus edition

 In the scene in HBP, Voldemort is around 35 years old  by my 
calculations and Snape is eighteen years into the future.
So Rosier is at least 18 years older than Snape.
How could he have been a part of the Slytherin gang then?

Also when Voldemort is trying to get Slughorn to tell him about 
Horcruxes, Slughorn tells Lestrange and Avery that he wants their 
essays which shows that at least one of the Lestrange brothers and 
Avery were at school with LV. So how could they have been part of 
the Slytherin gang Snape consorted with? They had already completed 
their education by that time.
The only member mentioned in that passage that Snape could have been 
at school with is Wilkes.
So is Sirius lying here? Is he talking about the people Snape hung 
around with while in school or after school was over?

The more I look at those two passages I think Snape did not have any 
friends in school. He was not in contact with future DEs anyway.
He could not have hung around with Lucius since he is 6 years older 
than Snape. That leaves Bella and Cissy.Highly unlikely IMO.
Anyway I'm confused. Help!

And as a side note is Rookwood's name Augustus or Algernon?
He is referred to as Augustus in GoF, but in the Daily Prophet, it 
is Algernon Rookwood. Siblings, or the same man? Or a FLINT?

Elyse