I sleep trained my son at 5 months. He’s 3.5 and the BEST sleeper. Literally perfect. My daughter has been harder. Sometimes I have to let her cry it out, like last night for 15-20 mins because if I go in there, she gets more upset. She didn’t do well with Ferber.
I don't have to do the full sleep training again, but I feel like there's 3 or 4 days every few weeks where it's harder to get him to sleep/he cries a lot and fights it so I kind of sit by the crib until he sleeps and that seems to work well enough now. But hey, it's better than before when he was waking up hourly. Or when we coslept and never had enough room in bed 😂