Peanutter just reminded me - I was 5 when my brother was born and I clearly remember changing nappies (cloth and pin type!), he was wriggly but not mobile so he must have been around 6 months and me 5.5 years. I also warmed bottles/fed him (with instruction) and carted him about the place. I would trust my DS to do these things when he is 5, he is nearly capable now.