If you are wanting to set different aggro for differnent mobs even of the same level, (you can use MySQL statements for this but I use MySQLFront because it is a point and click thing) just find the ID of the mob in question and change the aggroradius of it. Or in MySQL statement something like this (assuming you want to change mobs with the ID of #1)
Code:
UPDATE npc_types SET aggroradius=65 WHERE id=1;
The solution Monrezz gave is a good one, but is a generic fix of aggro accross the board, meaning that ALL mobs lvl 65 will have a agroradius of 100.....And the way I am telling you, is VERY tedious, as you would have to change 71775 (and climbing) records....