Baby yawns

Today I sat next to a baby for like 1/2 hour on the subway. I made friends with the baby, talked, made faces, let it hold my finger, and then decided to do some experiments on it. You know how if you see someone else yawn you can't help but yawn yourself? I decided to see if this works on babies. I yawned several different times while the baby was watching me, but the baby never yawned back. Which raises the question: at what age to people start yawning back?