No, they could not. That would be ridiculous, because that would mean that mages at level 20 would all be able to cast 9th tier spells, which is not only crazy, but also terrifying.
If you want to go with D&D logic, think of the level difference between the two settings. The typical max level for D&D is 20, whereas the max level for Oubliette Online is 120. As such, it would be reasonable to assume that you get a new 'tier' of magic in OO much slower than you do in D&D.
Of course, I don't want people to have to feel like total scrubs, you guys [i]are[/i] supposed to be slightly stronger than average run-of-the-mill rank-and-file soldiers/wizards.
So, in the case of spellcasters, imagine that you get a new 'tier' of spells every 8~10th level, depending on how innately talented you are.

Alternatively, we could do something like magic tiers being progressively harder to reach, with tier 1 being available to any wizard of level 1~5, 2nd tier to those of level 6~15, 3rd tier to those of 16~30 and so on and so forth. I'm open to suggestions and ideas of course.

[b]EDIT:[/b]

As [@Chiro] said. We're not bound to the same restriction or system, but at the same time, things have to be  fair and reasonable. In a game where the max level is 120, it would be strange if you could do a spell like "Call Meteor" when you're only level 20 and haven't even come close to the end-game content yet. :P