What age is too young to sleep on different level?

rlc87

My son will be 20 months when DD is born in November. I’m trying to decide best sleeping arrangements. Baby #1 is a great sleeper, but a light sleeper. I feel like baby #2 will be waking him up constantly whether it’s naps or nighttime if they share the room. I do have a guest room downstairs( we have a 3 level split) I’ve been thinking about moving #1 downstairs, but I worry that he’s too young to sleep on a different level. What are your thoughts on it?