Just wondering how many months onwards should a baby stop using yao lan?

For my gal, I stopped using when she was 9 mths old. I went back to work when she was 7 mths old and taken care by helper. Stopped when I saw helper sleeping while rocking my child in the yao lan, so I stopped using for safety reasons. For my boy, he totally rejected it when he was 3 mths old. Thought is easier for my mil but my boy didnt like the rocking momentum and prefers cot.