How old did your babies/kids start sharing a room?

We live in a very small house. Our March 2019 boy has enjoyed his own room since about 8-9weeks old. I’m due with little brother in the next 2-3 weeks and trying to think ahead. When to put the new baby in with his older brother? We don’t have enough rooms for everyone to be separated. I don’t want to ruin older brothers sleep but my husband is a light sleeper and our first was a very noisy sleeper as an infant.

Older brother is almost 18m and sleeping in a twin bed with toddler rails and room is fully baby proofed with a gate at the door. Crib is still in the room too. He mostly sleeps through the night.

Thanks for any tips or advice