Congrats for waiting this long - many parents don't.
Honestly, this will depend on your child. If they are prone to addictive or obsessive behaviour, a smart phone will only amplify the tendancy. We already know how hard it is for adults to put down their phones for any length of time, and kids typically have less will power.
That said - digital communication is an important part of most people's lives now. If all her friends are using a particular app to communicate, they will "need" it too. Some parental controls would be good for the first phone -- which apps get installed, etc. Just be prepared to unlock most of them. ;-)
You might want a phone "lockbox" at home to ensure they turn off. Hopefully the school is strict about phone usage and etiquette too - it can help.
Did you find a solution?