So, we found Norman in the garden with a broken leg in September. He came to live with us in October and was on cage rest until December. He is appox 1-2 years old (we think closer to 1 as he has grown loads)
Old lady cat has been with us 14 years - so approx 16. Her life consists of sleeping, eating, sleeping and sleeping some more
When N was first let out, she ignored him. He tried, what looked like, playing with her. She was having none of it and would hiss and growl if he walked past. He would then leave her alone.
For the last few weeks, he has been getting bolder. He will jump on her and bite her neck. She always had the upper hand and would see him off quite quickly. But now he is getting braver and really going for her The scraps can be quite spectacular, she isn't managing to see him off for a good 30 secs
We have been shouting no and pulling him off, then ignoring for a few minutes. We've upped playtime, to wear him out. He is very playful chap, always climbing places he shouldn't. He has huge character
He is worse if he's hungry, but he is always hungry! He would eat all day if we let him. They both have 2 pouches of wet food and a bowl of dry through the day - we have to feed him at set times as he doesn't know how to limit himself so can't leave food down and also, OLC is getting fat! He is still very lean (was a bag of bones when we found him)
So what do we do? OLC is obviously unhappy at being attacked. We let him out in the daytime and this gives her a break. We can't convince her to sleep in a different room out of his way, she likes the lounge windowsill or footstool. I feel sorry for her and him so really need to try and sort it for them both, it's not fair on either of them
This sounds a bit like our situation at home. We had 2 very old cats - 18 and 17 years old, the 17 yr old is blind (18 yr old has since died). We then took in a little stray kitten, at first he was very timid & quiet but from about 12 weeks old he began jumping on the older cats and playfighting with them. There was lots of yowling from the older cats and it was particularly distressing for the blind cat as he didn't get any warning of the ambushes. When the kitten was little he could be batted off easily but as he grew bigger & stronger he was able to hang onto the oldies. We did what you are doing - lots of play with the young one, letting him outside to run his energy off, separating them, distracting him...
As he has got older he has become less likely to ambush the remaining old boy (he is 10 months old now) and instead he spends a lot of his time attempting to groom his old friend (old boy submits to this - my other old girl cat would not have tolerated it)
Maybe he is a little bit younger than what is estimated - so he may get this out of his system and settle down a bit