your child is five or six months old and before he is crawling and walking.
keep the gates up until you are convinced that your child can walk up and down the stairs safely, usually at around three years of age (after which time he will likely learn how to bypass the gate anyway).
baby safety gates are also a good way to keep your toddler in his room once he goes to bed. if he wakes up in the middle of the night, a gate will prevent him from being able to come to your room, which typically creates a habit of continuing to wake up. 