Dear A.,
Both our dentist and orthodontist recommended that our daughter get her expander before puberty as the bones are not yet fused at that point and would be much easier to move. That was the best advice we ever got and I'm so glad we did it when we did! It seemed crazy to me to start orthodontics that early, but it took so much less time and was not painful at all. I remember going through braces at 15/16 and it taking years and years and being excrutiating. My daughter experienced none of this pain and each "phase" of the expander and braces took just a few months.
My daughter had been a thumb sucker and this made the roof of her mouth "tent up". As a result, her face looked pinched. The expander changed the entire shape of her face. She was very pretty before, but is absolutely beautiful now at 12 (says her totally unbiased mom). And, a side benefit of the expander is that it cured the thumb sucking. My sister (now in her 40's) who hates the shape of her own face, was told my her daughter's orthodontist that if he was treating her now, she'd get an expander and look like a different person!
My daughter went through puberty early and had her period by age 11. I'm really glad we didn't wait, as I wouldn't have imagined that she'd mature that early. However, now when we are busy with all kinds of teenaged activity the braces days are behind us. She was much more cooperative with those things when she was younger!
So, again, trust the professionals, definitely do it before puberty, and follow the directions closely. You'll be glad you did!