At what age it is safe to introduce a comforter (with dummies attached) into the cot ?

It is important that there is no risk to baby’s head/face being covered by anything soft or loose in the sleep environment.
Red Nose has the following general information about soft toys/comforters.
Keep soft toys out of the sleeping environment for babies under seven months of age because they may cover the nose and mouth and interfere with breathing.
Some babies over seven months of age may appreciate a small object such as a soft toy to provide comfort and connection (transitional object) during times of separation from their parent.
https://rednose.org.au/article/soft-toys-in-the-cot