It depends on the child, of course.
We got my SD a phone at age 7 because she was at her mom's and her mom only had her own cellphone. She never answered it and wouldn't allow my SD to use it. So we got her a 4 button phone and programmed numbers into it. It made her feel better to know she could call us whenever.
That being said, WE had to be responsible for it because she was not yet ready. We had to constantly ask her where it was, and we had to put it in her backpack when she left and retrieve it when she came home.
We traded her 4-button phone in for a regular phone at age 9. However, she was again, not really ready for the responsibility because she used the phone to prank-call relatives when she was bored. She didn't know that we could SEE her phone number, and that her father and I could check and see everything she did. We got Parental Controls on the phone that made it impossible for her to use it for any numbers except ours. Later we had to change the parental controls to turn the phone off at night when we found her texting friends late at night. Most of her friends got phones around age 9-10 so she was starting to text friends.
At age 11 she is now fully responsible for her phone. I do check all her text messages from time to time but she has treated the phone well and not lost it. She no longer does things she shouldn't on the phone and I think she's set up to be a responsible texter and phone user.
Depending on your daughter's personality, 8 can be okay for a cellphone. Otherwise just know that YOU are going to have to help keep track of it!
I would also like to mention that even though we got my SD a cellphone, she never answered the thing! She just kept it in her bag and went on playing and such. Now she STILL doesn't answer it half the time, she's too busy being a kid. It gets more use for HER to call US. And she doesn't think to check her messages. It's not on her mind to keep the phone handy, or listen for the ring.
The only reason we continued her having a phone was because we signed a 2-year contract the first time, and then when she was 9 we signed another 2-year to get the free phone upgrade. Also there were issues at her mom's house and she DID need to call us a few times. For us, the cost was worth it for just those few times she needed to call us.
Your daughter will probably not need the phone, nor need to use it yet. If my SD lived here all the time she probably wouldn't have gotten a phone until she was 11-12. It's only now that she's getting lots of use out of it and can finally be responsible for it.
Hope that helps!