I fostered a litter of puppies which included my black Labrador, Captain, and taught them all to “sit” at 5 weeks in age. By 12 weeks in age they never jumped on people!
Inconsistency in humans can slow training! Sometimes, when we yell at our dogs to “stop” this can actually reinforce the unwanted behavior because they think to themselves, “I’m getting attention!”. Dogs need to know what it is you actually want them to do.
Remember consistency is key! Have treats handy at the front door. As soon as you walk into your home ask your dog to “sit”. If they perform this cue you can give them a treat. Eventually they will learn that when they “sit” they will get treats and that jumping does not.
For more information, check out Truly Force Free Animal Training’s Good Manners Course here: https://trulyforcefree.com/course/truly-force-free-good-manners-course/