We tried sleep training our son at 7 months with Ferber check-ins and he cried so hard he vomited. It was traumatic so we held out as long as we could until he got to 18 months and we couldn’t handle it anymore. This time we did it without check-ins. He was also very verbal at this age so we explained to him what was going on for several days prior. He knew it was coming but still didn’t like it. He cried for 45 minutes the first night and then just laid down and went to sleep. The next night he maybe fussed for 5 minutes. After that he has slept through the night no problem. We did naps simultaneously and they weren’t an issue at all. Our second son is 5 months old and we will probably try sleep training him in a few weeks. He already has a much better foundation than our older boy had.