Although babies don't usually recognize their reflection until they're around 18 months old, adding a baby & kids mirror to their room can still make a huge difference:
It stimulates younger babies, helping them familiarize themselves with their reflection and develop their self-awareness at later stages, too;
It encourages them to move closer to the mirror, reach out and point to what they see in its reflection, allowing them to refine their hand-eye coordination;
Playing with their reflection will also grow their attention span and help them focus on a task or object for longer;
It's a creative way to teach them gestures and vocabulary through imitation;
Once they're a bit older, a mirror can help them get familiar with their outfits and clothes when they are getting ready in the morning, allowing them to be a bit more independent.