If I add my bonus ability point gaining by achieving the 12th level, to an odd intelligence score, should I gain retroactively a skill point for each past level?
Per the SRD:
A character does not retroactively get additional skill points for previous levels if she increases her intelligence.
In 3.5e, you do not get retroactive skill points, only starting on the level you gain the int bonus or the level after if the permanent bonus was from something other than levelling.
In this discussion, James Jacobs explains why Pathfinder chose to do it differently.