This is difficult for me to answer tbh, as all the games consoles in our house belong to me and Dh not the DSs. They've been playing with them from quite an early age, but mainly because they see us playing with them. If we had no interest and didn't already have them , maybe 7 or 8 would be the age they start picking things up about them from friends? So, maybe a few years after that before you gave in and got them one?
DS1 got his first gameboy when he was 5 (present from overseas SIL) Obviously we didn't let him play it 24/7, and we are quite strict about the games they have, but we don't get all twitchy about kids and video games anyway.
If they become total nerds and technogeeks, at least I don't have to worry about them getting socail lives, and getting involved in sex, drugs and rock and roll