zone/exp.cpp - Client::GetEXPForLevel is where you would normally adjust this. Be warned however, that the first time an existing player logs in and gains xp, they will level or delevel to match whatever you've set up. I evened out my xp curve and set up a player script to fix this, but it's not especially elegant.
For non-retroactive scaling you could do some manipulation in zone/attack.cpp, NPC:
eath and change how much is actually given per mob or client level range. I did this to expedite a few early levels and it works out nicely.
The only database method I can think of right now would be to assess your zones for ideal level range and change the zone multiplier of zones where the ranges you want to alter would play, but then you'd probably have people farming guards in low level zones. (I'm pretty sure that's what blackdragonsdg was suggesting.)
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