At what age is it safe to introduce a comforter for a baby to sleep with? And is there a style that is safest? Thanks

Red Nose recommends that you do not introduce a soft toy/comforter to babies less than 7 months of age.

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.
On balance of the current evidence, the risk of suffocation posed by the presence of soft toys or objects in the baby’s sleeping environment outweighs any benefit to the baby from the presence of a transitional object in the cot. It is therefore advised not place soft toys and other soft objects in the cot for babies under seven months of age.
https://rednose.org.au/article/soft-toys-in-the-cot