From holding baby to sleep to making him sleep independently?

For mummies who didn’t sleep train, how old was your baby when he or she managed to fall asleep independently without you rocking them? My baby is 3 months old and I’ve been rocking him to sleep and co-sleep as he wakes up shortly in his crib. For naps, I’ve been holding him so that he can sleep longer. But I think this isn’t good and sustainable :(