Temporary mouth breathing can happen with a cold, but chronic mouth breathing is different. When it continues over time, it should not simply be assumed that a child will outgrow it without consequences.
The short answer
Some temporary mouth breathing improves, but chronic mouth breathing should not be assumed to go away on its own. If it persists, it may affect sleep, posture, and facial growth.
Why it happens
Persistent mouth breathing usually has a reason, whether structural, functional, or inflammatory.
Why it matters
The longer it continues during growth, the more influence it may have on development.
What most people miss
Waiting may allow the pattern to become more deeply established.
What to do next
If it is chronic, have it evaluated instead of assuming your child will outgrow it.