I am applying for ancestry visa and it is asking for my address. My passport/id say my old address but we have recently moved and not updated out ids yet. Should I keep my old address on the application?
No. Put your current address. It's more important for them to know where you live and where they can send things to you than it is for the address to match the one in your passport. It's entirely normal for people not to live in the same place for the entire life of a document with long validity such as a passport.
If South African law requires you to keep your passport up to date with respect to your address, then you may want to address that.