I think between 3-5 yr will be the best.
My daughter is 4.5 now and I am expecting next month. As she is quite independent, she wont ask me to carry her or she can even tell me to eat well, otherwise the baby will starve inside!
She even said she can offer me help when the baby comes. We bought a baby rocker chair and she said she will rock the baby as well as clean the diaper! I am not sure if kids understand all those if he/she is under 3yr?