I hate to say it, since I expected Night Elf mages to be arcane powerhouses myself, but in-game content trumps speculation.
The Azshara questline is pretty definitive. The Blood Elf in question mocks the Night Elves, questions their old, faulty scrying-stone "hardware", and laughs out loud at their choice of using those arcane constructs from Dire Maul as combat pets. Apparently, great as those things are, you just don't use those for combat if you know anything about magic (Khadgar using them as tour-guides is probably still coo', though). Unlike the typical water elemental, those things are really temperamental and really easy to turn against their caster. He does exactly that, hacking an arcane construct and melting the whole lot of Night Elves into goop.
I was expecting a "oh, they're actually not bad!" moment in that questline to break up the stream of "Night Elves? With Magic? AHAHAHAHA! Oh, that's adorable, they're so cute!", but it never came. He winds up completely justified in his opinion that watching Night Elves study magic is like watching children play with firearms. The Night Elves aren't just bad because they're all rookies - the very idea of using arcane constructs is bad, and that has to be something passed down by the High-Borne. It's pretty obvious that most Night Elves just aren't that good at magic yet, and even the original High-Borne are using and teaching "outdated" techniques (and the reception on their scrying stones is just awful!)
Warcraft magic doesn't really seem to be linked to age - and it really is pretty science-like when you get right down to it. It's all about theories and formulas. Things like that don't get better with time - they get outdated. It's not so much about collecting the wisdom of the ancients over the long centuries - but experimenting and doing new things the ancients haven't tried. Jaina and that guy I don't wanna mention are two of the strongest mages around - and they're humans. Pretty young humans, too - so you don't even need to be old-by-human-standards to get really good at magic. Khadgar might be stronger - but, again, he's only old by human standards, and in his glory days, even he was, if I remember right, not actually old but artificially aged by Medivh's curse. Obviously, young people with new ideas or just a natural aptitude can and regularly do surpass the old masters. Blood Elves might not live as old as the High-Borne have, but all signs suggest that learning the new magic theories a hundred years ago is much more effective than having ten thousand years to work and refine the old ones.
In short, it'd certainly make sense to roleplay as an arrogant High-Borne who thinks his ancient lore and centuries of wisdom make him superior to anything the "young races" could possibly do magic-wise - as long as you keep in mind that it isn't actually true. Your High-Borne is going to be in for a very, very unpleasant surprise the day he comes up against an experienced Blood Elf Magister.
Edited by Sylassanna on 11/28/2010 3:00 AM PST