The protectiveness of sow bears towards its cubs is generic across all the types of bears. There might be a difference in the aggressiveness of a type of bear but this is purely based on how comfortable a bear is with a human and how threatened it feels. eg. a black bear has a smaller circle of fear and hence one can get closer to a black bear than a grizzly or an asiatic bear.
Having said that, bears attack on humans occurs due to multiple reasons:
- A bear feels that the human is a threat to its cubs or itself.
- A bear is startled by a human.
- An incentive - say food carried by humans (Most common when camping in bear area).
Most of the times, sow bears try to avoid conflict as it can lead to its cubs being killed. Bears with cubs turn aggressive only when they feel that they cannot escape (A person being too close or the bear being startled by an unsuspecting human). This behavior is not specific to types of bears or even seasons or age of the cubs. This continues to be the behavior till the cubs leave the mother (typically after around 2 years). Sometimes the search for food might cause a sow bear to wander near to the human settlements or an unsuspecting camper. Still, the rules of attack are mainly guided by whether it can escape with the cubs or it has to fight for the survival of the cubs.
Always remember that winter seasons, though a time for hibernation, cannot guarantee a sow bear to be docile(In fact, bear hibernation is not a pure hibernation and is called as winter lethargy. And hence it can wake up much faster than true hibernators). It's always better to keep distance from a bear with cubs irrespective of season/age of the cubs/type of bear.