Assuming Slimer had a regular mortal life, there's also the question of whether he actually remembers it and how old he was when he died. He definitely seems to have a childlike personality in the series, as emphasized in episodes such as Once Upon a Slime, when Peter mistakes the dessert magazine for Playboy or something similar and says that Slimer is too young for that. LOL. (I am amazed they got away with that joke, especially since that was after the retooling started!) Actually picturing Slimer as having been a child when he died is pretty sad, though. I tend to picture him as having lived to adulthood, but then it's a toss-up whether he died young or not and whether his personality by the time of the series has much bearing on what he was like when he was alive.
So, has anyone else been curious? Do you have your own headcanons, either vague or fully fleshed-out? Do you prefer the series leaving it a mystery or wish it had been explored?