It's often mistakenly referred to as hay fever, because it has nothing to do with hay, and there's never a fever. The term was coined in 1828 by a British physician, Dr. John Bostock, who suffered from allergies and noted a correlation between his symptoms and the British haying season. Of course, allergies are tricky. This would explain the ebb and flow of your symptoms. In fact, one of the reason allergies are so hard to treat is because no two people suffer exactly the same.
Nobody knows for sure.
The immune system is filled with mysteries we're still trying to uncover. And the tendency to get allergies is definitely inherited. Allergies usually begin in childhood. But they can develop at any age for the first time. It's not unusual for new symptoms to develop in people in their sixties.