We did relatively young, but it really comes down to when you can trust that they will hang on consistently, stay firmly on the saddle and they won't try to get off when you stop - or worse slow down.
I don't know how much your 8 year old weighs, but if you are getting it for the other one there would be no harm in trying to see if he could try it a few times. Getting comfortable with the leaning required for biking is a huge help in eventually learning to ride. For some uncoordinated kids it also helps to get the pedaling action down.