Nasonex is geared towards allergies. It helped my son a bit but he has had his adnoids out too, they were HUGE! Both of my kids had adnoids out.
I sent you a side note with the information you asked of me, hope it helps.
It helped both of my kids get better sleep, my daughter snored or didn't sleep well do to sleep apnea and removing her adnoids helped tremendously with her. My son we went to the ENT today and actually are going for allergy testing just to rule out as the Nasonex isn't helping with the sneezing, watery eyes, hoarseness on occassion or congestion.
The surgery in itself is easy, quick and under an hour. It is scary, however for me it was a positive having it done as both of mine got relief with sleep apnea issues.
Keep her dosed with saline drops too, this dry air here does not help matters at all! I do it before they go outside, when they come in, before bed as much as I can as having the dry air can cause problems all by itself.
I don't know about it being common for age, I think it is more genetic as I had enormous ones, my daughters were too big for her petite size and my son's were big.
Good luck!