The older dog is not attacking the younger one out of nowhere, although it seems like that to you. there is a reason for it even if you don't perceive it.
Do not let the dogs be together without supervision in the house, ever.
Then, do little sessions with the dogs together in a very controlled manner, as follows:
Remove all toys and food so there's no hassle over those things.
Keep the older dog on a leash, or the younger one if that is easier, or both if you have someone there who can help you, but make sure you have control if something starts to go sideways.
Do this at a time when you can watch the dogs 100% of the time. Not while texting or talking on the phone or reading a book or being online.
Watch the body language of the dogs very carefully so you can learn what it is that is setting the older dog off. This may take a while to learn if you are not familiar with dog body language but it's fascinating to learn, so dedicate yourself to it. It might be when the younger dog moves in a certain way, or flicks his ears even. There's something going on here that you don't see....train yourself to see it.
Keep these sessions with the dogs together and leashed to 5 minutes at first, no longer. If one dog gets upset, end the session of observation and separate them.
The rest of the time, keep them separate so they don't get into a tussle. But do these 5 minute together times two or three times a day.
While you are doing this, the entire time that they are both being calm, they should be praised and given treats and talked to in very nice encouraging ways. If one gets upset or the older dog tries to attack, do not pull or yank the leash, don't scold or get angry at all, simply remove quietly and calmly them from each other's presence.
Do this for a few weeks and see if progress is made.
Best of luck!