Engkau/kau/ko commonly are use for friends or among siblings .. but then, 'kau, aku' words are actually a common word for slave in long time ago but still use it until now when it come to pray between God and Men .. anyhow, it also commonly use in song lyrics and yes it is sound quite rude if you use for addressing elders.

Never use "aku" and "kau" to older people ;)