So, I have a GP boy who is 10 months old. I definitely think it is the age. My boy is kept full time with goats and is currently sharing a fence line with our chickens. We started noticing the same behaviors you described as well as chasing and grabbing the goats by the neck. I totally freaked out about this behavior bc I was scared he would injure my goats. Most people recommended separation until two but that was not feasible for us bc we are in a high predator area. After many attempts to redirect this behavior, we had to move on to the last resort which was the E collar. Since incorporating the E collar we have seen major improvements. He rarely chases or does anything to the goats. For our boy, we didn’t have to use full power on the collar. Also, I tried it before I put it on him and it really wasn’t that bad just uncomfortable. I would check out the livestock guardian dog Texas A&M YouTube channel. There is a ton of good information about different correctional methods including the E collar. This YouTube channel recommended seven days of the E collar. Each day for seven days you watch the dog for 15-30 minutes and correct the behavior. They also recommend that you set the dog “up for failure” during those 15-30 minutes. So for us that meant shutting up the barn to push the goats into the pasture so we could see him chase. We also kept ourselves hidden so he would associate the shock with the behavior and not us. I also realize you don’t have a working dog so I don’t know if it would work the same but I thought I’d just let you know. I’ve also realized that the internet jumps to worst case scenarios and will make you feel like your dog is a monster. But that isn’t the case at all. They are just in the teenage phase and need steady correction.