What, exactly, is wrong with having long-term goals? It is one thing for a first level character to state "I want to be a powerful necromancer" it is another to state "I am a powerful necromancer." I have no problem with the first but I would with the second.
Are all PCs 18 years old when a campaign starts?They sure are not in my games. My current character (the one I referenced a few pages back) was 45 at the start of the campaign.
If your PCs are not young at 1st level, you are an exception and obviously my statement about an 18 year old does not apply.
For 18 year olds, most don't have far reaching plans because as Dr. Phil says, they're brains ain't finished growing yet.
Such statements of "I want to be a god" or "I want to rule this empire" or "I want to be a necromancer lich" are the statements of the player's long term goal. For a PC, such statements for most people would be out of character.
To me, it's more plausible to develop into such a goal through game events, rather than declaring pre-game that he has this high and lofty goal to pursue in a game world he hasn't even seen.