Every pup is different and it's up to the handler to gauge when they're ready, I would usually start at 9-10 months but the one I've got now was over 12 months before she was mentally ready, infact for a while I just didn't think she had it in her, thankfully everything has now fallen into place and she's doing what's required.
Must add I've never been one for taking a pup out on the lamp with an older dog, even if it's to let it watch. Always just took them out on there own to start with, just me and the pup which ensures there are no distractions, other dogs present can also cause issue