I'm pretty sure they can have boiled water from birth. I started my bub on it at 5mths but the only reason i did so late is because i am exclusively breastfeeding her (she won't take the bottle at all) and that is when she started to use the cup. She is 8.5mths now and loves her water. They say it's better to hold off on juice though as babies get addicted to it and wont have water, plus juice is bad for their teeth (even when teeth have not cut though yet!). Hope this helps - your bub is definitely old enough for boiled water.
Sherri, mum to Ella Bella Baby Bear ~ 21.01.05