Yes, you can have your first panic attack at any age and yes it can come out of the blue. Perhaps the anxiety / panic was bubbling under the surface and broke through (OCD and anxiety / panic run together often). When looking at my life retrospectively, I've experienced that "break through" axniety. Once it breaks through, you can continue to be panicky for a while thereafter.
Good that you're going to have heart issues ruled out. If you do have panic / anxiety, there are a lot of good people and resources here to lean on.
Hang in there!
Anxiety Sufferer Since the 1980s
Lexapro @ 10mg per day
Trazodone as needed for sleep