I'm not sure and I've had lots of thunderphobic dogs. It does get worse with age. I had a Dobe who achieved her WAC with no problems, then as she aged and we moved to Houston she got very thunderphobic and she was an only dog when this happened, so she didn't learn it from anyone.
I've also had them get better. Austin was close to ballistic when I got him and over the years, he got better. Found his safe spot and he would go there and he was fine.
Poor Tootsie was the worst. She did like the CD and by the time we got to the third song her Calm Spirit was kicking in and she was ok -- as ok as she could be.
Right now Batman is getting better and Isaac is getting worse.