This is such a personal thing, there are so many people who are totally against things such as the DS, but here are my thoughts.
My son (nearly 7) started asking for one at around age 3 or 4. Now, he has uncles and one aunt that are very close to his age (10 years is the biggest age gap there is) so he was perhaps more aware of them because of this. Also, some of the other boys a few years older at daycare had them. My husband and I told him no, that's for older kids. When he turned 5, the onslaught began. I told him flat out that he wasn't old enough for it, no matter who else had them at that time.
He started kindergarten last fall, and asked for one for his birthday that month, and for Christmas, and again we said no. However, this was more a maturity thing and a wanting to keep him focused on school thing that an aversion to it or thinking he was still too young. In April, we sat him down and told him that if he got all "S"es on his report card at the end of the school year, then he would be rewarded with a DS. He did, so he got one a week after school was over. We sat him down, gave him the rules (no playing in his bedroom, everything kept together when not playing, when we say turn it off, do it or you lose it, etc...) and so far there have been few times that it has been an issue. He spent last week camping with my mom, we sent it because of the distance they were traveling, and I was a little worried he'd want to play it more than he should because he wasn't at home, but when I picked him up my mom said he maybe played with it twice over a 5 day period.
He was ready for it, but all kids are different. Hold out longer if you can and if you want to; set some goals for her to "earn" the system, that makes them more invested in it.
One more thing, it seems that girls are less likely to be overly interested in it at a younger age. I see some girls (like my sister) who have them, and play with them, but not that much. I would keep that in mind as well when making a decision.
Good luck!