How old do i need a monitor in my child's room?

Never necessary: Monitors are a convenience product for those who want some way to be forever present with baby. For those who can't put baby in their room without one, that's better than the alternative so parents & child can sleep better. However, i would never buy one or recommend one. The best place for them is to be left at the store or in the box.

Baby moniter: There's really no set time, but I would say when the baby starts sleeping through the night, and there are no more night feeds. If you are a deep sleeper, or in a far room; if you're afraid you'll miss them crying, longer. If you keep it, turn the volume down, so you can get a good night sleep, low enough to only hear crying. Some moms keep it till kids are in a big bed, and come and get them.