3 of my DGC have asked to learn to knit and I said they should wait until they were 5. Then they got it slowly, but they don't get much practice as I keep their knitting and we don't see them that often (I keep it as none of their parents are knitters and so couldn't pick up the dropped stitches or get rid of the extra ones which appear from nowhere!).
For some reason, and it isn't practice (see above), DGD and DGS2 both suddenly got a lot more proficient at about the age of 8,while DGS5 is still very slow at the age of 6. I do think, therefore, that perhaps 5 is too young, but on the other hand, you don't want to put them off if they want to try.
I used much the same method as Gagagran, although had never heard of the bridges, and so just said what I was told by my mother, "in, over, through, off".