A.R.
We have a Wii and both my kids have a DS - ages 8 & almost 7.
The major difference between them is portability - you can take the DS anywhere. The boys (8-9) in my neighborhood will take them outside and play Pokemon; they trade characters, help each other through levels, DS download (text each other/play same game together) etc etc - then when they get bored with it they grab the football and play football. I was really concerned they would play it all the time, but it's not been an issue. One thing to watch out for with boys is the rating on the games. Because it's portable they take it out to play with their friends. My son is only allowed to play E rated games, but his friends can play T & M rated games so he has access to them when they hang out and play together. He knows he'll lose his DS if he plays them, and it hasn't been an issue, but it is something for you to consider.
My 6 year old doesn't play with hers that much, but she's never been a gamer type. My son had a DS lite at 7 and loved it as well, I think it's totally appropriate for a 7 year old as long as you supervise the time spent and games played.