Any Depth Works
According to inspectapedia.com, no single answer exists for the question of septic tank depth. Just about any depth is acceptable. Even a partially exposed septic tank is unlikely to freeze in winter because of two factors: ground temperature and "heat generated by the bacterial action going on in the septic tank."
You May Have a Preference
Do you want your septic tank to be above-ground, exposed for all to see? Probably not, especially in more urban settings where space is a major concern. Since, according to inspectapedia.com, a septic tank's functionality is dependent on gravity, the tank should be lower than the lowest sewer line to which it's connected.
No exact answer exists as to how deep a septic tank should be. Because of its dependence on gravity, however, the tank should be buried with the sewer lines in mind. The further a tank is from the building it serves, the deeper it must be buried for the sewer lines to move waste to the tank.