They used to nap in a heap. Now the younger one pesters the older one from breakfast until bedtime, the play gets shrill, and someone yelps. You spend the evening refereeing.
For ordinary rough play and pestering, this is a management problem with a short answer. For anything that looks like real conflict, skip to the bottom of this page now.
Why the balance tips at this age
The younger dog is now big enough and fast enough to be genuinely annoying. Their impulse control is temporarily worse than it was at 4 months, so they start things they cannot stop.
Meanwhile the older dog has been absorbing this for weeks. Dogs who cannot get away from a pestering housemate tend to escalate what they say, because the quiet versions were ignored.
The thing most households get wrong
The instinct is to let them sort it out. What that produces is many hours of practice at getting overexcited together, which is the skill you are trying to reduce.
Two dogs in one room are also each other's reward. Every time the play restarts, it pays immediately, so it restarts faster next time.
Today: end the sessions before they curdle
Time one play session. Most households find the tone changes somewhere between 90 seconds and 3 minutes. Write down your number.
Interrupt at half that. Not with a shout. Walk in, scatter 10 pieces of food on the floor 6 feet apart, and let both dogs sniff it up separately.
Then separate for 20 minutes. A closed door, a pen, or one dog on a leash beside you with something to chew. This is the part that does the work.
Relaxation is trained as a behaviour in its own right, in short repeatable sets, below the point where a dog cannot think. Enforced downtime is that, applied to a household.
Give the older dog a working exit
One raised bed or one room the younger dog cannot follow into. Not a corner they can be cornered in.
When the older dog leaves, that is the end of the session. Do not carry them back in, and do not let the younger one go and collect them.
Feed and chew separately, every time, behind a door. Resources are the easiest thing in the house to remove from the argument.
Train them apart, not together
Whatever you are working on with the adolescent, do 5 minutes of it with the other dog somewhere else. A dog who cannot think with their housemate in the room is not learning.
Walk them separately twice this week too. Most owners find the adolescent is a noticeably different dog on their own, which tells you what the walk problem actually is.
When to stop and get help now
A bite that broke skin.
A fight you had to physically break up.
Either dog freezing, or hard-staring at the other.
Guarding food or a resting place with a growl.
One dog no longer eating or sleeping normally.
Safety note
If any of those is true, keep the dogs separated and book a veterinarian and a qualified behaviour professional who can see both. Sudden behaviour change warrants a veterinary examination, and pain often presents as a change in behaviour.
Today's version, in one line: time one play session, interrupt at half of it, then 20 minutes apart.