DS (just 10) and I made a plane at the weekend,(small one,spitfire sized),and he could have probably done that on his own if he'd done it before.But it was very fiddly,and it was more fun with the 2 of us.I think it's a balance between what their fine motor skills are like,how perfectionist they are,and how much they actually want to build the model.(This was my model actually but Ds appeared and asked if he could build it with me!)
Its a good idea not to be too ambitious at first!