What's the perfect age gap to follow our eldest?

There's no perfect age gap. It's all up to you. With a close age gap, they will be more like playmates whereas for siblings with a large age gap, the elder one will play the role of a mentor and guide. Doctors recommend that the mother rests her body for at least 18 months after pregnancy before trying to conceive, so if you count 9 months after that, the recommended age gap should be at least 27 months between babies.