Yesterday, I talked about how magic is the defining factor that allows a 0-level character to gain class levels. No, this doesn't happen EVERY time a person is exposed to magic(though a DM in a High Magic campaign could allow it), but often enough to allow a few relative nobodies to band together to eventually become heroic powerhouses.
Listed below are a few ideas on how magic can be used(and in some cases HAVE been used) to explain why the PCs have class levels as part of their backstories.
- Race: Some races; Elves, Gnomes, and even Dwarves, are often portrayed as being innately magical, hence the higher incidence of classed characters in their societies as compared to human societies. You don't even have to be a full member of the race(such as half elves, or just a human with elven blood somewhere in his ancestry) to explain this.
- Magical Training: Most campaigns handle wizard training in 3 ways, either they're played in a fashion similar to 3rd edition sorcerers(magic is their birthright thanks to some sort of magical bloodline), They function on a Master-apprentice system, or their are magical colleges that young aspiring wizards may train at. Any way you do this, the wizard is exposed to large doses of magic and making them classed is the easiest of any class type out there. It also works well in settings where you want the majority of priests to be spellcasters.
- Touched by the Divine: This works best for clerics and Paladins, but there's no reason this couldn't work for any character. At some point during the character's life, he became exposed to divine power, perhaps an accident resulted in him being taken before a cleric for many days of magical healing. Alternately, perhaps the character was forced at some point in his life to call down the power of the gods directly into himself(perhaps to save a sibling or parent from a group of kobolds or giant rats).
- Trained by the magical: The Fae, Dragons, Celestials, Fiends, the undead, and various other creatures in the world are all magical in nature, and each one has skills similar to those of PC classes. It could be possible with DM permission to have a character trained by one of these creatures.
- Magical Locations: Perhaps their is more to that haunted house you went into as a kid. Maybe it had been the site of a terrible spellbattle in the distant past. Alternately, maybe the tree that you spent the majority of your time sitting under as a child grew from a seed planted in the remains of a dragon, or atop an ancient gateway for the Fae.