Your situation is why usually the rule of thumb for finding good companion cats is: same gender, same age (more important if they are young) and (more important if they are adults) same character/playstyle/social needs.
In your case, it sounds like they got along fine socially and character-wise, but the age discrepancy was a bit much. That is fine when both cats still have the same activity level, but in the long run it shows as your older cat isn't up for as much rough play (which male cats tend to be into, so that's perfectly normal behavior from the younger cat) as he was before.
There are two possible (very general) causes for this:
1) Your older cat is just that. Older. He is just not that into wrestling anymore and is showing that to his buddy...
2) Medical reasons. At your older cats age (although not ancient), it is not beyond reason that he does have a few aches in his joints. Does he jump less often or high than before? Maybe get a geriatric blood panel and have him x-rayed (to check for arthritis, spondylosis etc.) if you can at all afford it.
In either case, the younger cat will be getting more and more frustrated as he can't get rid of all that energy that he has and therefore may start ignoring the older cats signals of 'hey, thats enough now, I don't want to'. And while they will still generally like each other, this usually leads to their relationship getting worse. While you can step in, by 'just happening' to calmly need to go where the two of them are or making an odd/unknown (not loud) noise (yelling might send the wrong message and increase stress and energy rather than dissolve the situation), obviously that can a) only be done when you are present and quick enough and b) is a band-aid at best.
What I usually suggest in these cases (they are quite common) is to adopt another male that has had the chance to grow up with siblings/is social enough to accept a 'no, not now' from other cats but is the same age of your younger cat (4-6, not younger) and same activity level.
With social cats, these situations then resolves quite nicely as the two younger ones roll through the place in a ball of happy fur and the older one can join in if they like - but doesn't have to.
If this is not an option for you at all, the only other solution I would suggest is: give the younger one away and adopt a cat at the older ones age (as he knows and enjoys company in general, he just can't keep up with the younger one anymore). This is HARD, I know. It sucks and you immediately say 'oh gosh no never'.
Keep in mind that any other ideas (unless the reason for this issue is health-based!) will be mere temporary fixes.
My advice would be health check first, then third cat. If that's not possible, see above...