If it's a 1 on 1 I'm not fully sure, because you say their stamina is lower if they're older and nothing else changes, so if you could give a source on that because I wasn't originally under that assumption

The only old man we’ve seen fight without being affected by illness is Rayleigh, and the only thing that he noticed was off was his stamina(he hadnt fought any top tier in a long time), and Oda said in an sbs that he didnt like young people insulting old people and that in one piece he wouldnt like it to be like that, I’ll try to find his exact words

All that proves is that Stamina is hampered, it doesn't go against anything else it just confirms the stamina drain. The entire thing about Old People still being strong holds up even if they do grow a bit weaker, if you're an old Top Tier you're still a Top Tier just not the top tier anymore, not the way they were

We have nothing that demonstrates they get weaker, just that their stamina is heavily reduced, and from what we’ve seen old people do, its safe to assume they don’t

Sengoku had to stop Garp, or he would’ve actually been a threat to Akainu(probably there’s other reasons, but this is def one)

Sengoku could hold all of the BB pirates off alone

WB went crazy on marineford even being sick

Rayleigh stopped Kizaru for some time

Kaido and Big Mom arent the oldest, but they are in their prime still

Rare @Hereje015 W

The fact remains Garp = WB - young and old. Garp is still top-tier and BB main tactic is to negate df which he can't do to Garp. Garp is a haki monster and as @Hereje015 said BB takes double damage

As for the Sengoku stopping Garp part, either he was a threat or having the spiritual leader of the Marines, the Fist, the Hero of the Marines, go attacking an Admiral or losing to one would shatter morale.

Rayliegh Heald of Kizaru, Ace Heald of Kuzan for a time and Yamato has held of Kaido. You don't need to be a match in power to stall or hold off, unless you think Yamato is Kaido level that is.

Whitebeard did wreck absolute havoc no doubt but I don't think it's wrong to say he would pull of something more like Shiki if he was younger. Single-handedly breaking half of Marineford without a crew which is canon given chapter 0, he was another rival to Roger so Whitebeard should scale above that back then, but he didn't manage that alone, he needed his whole crew with him to break Marineford open like he did.

I think it's safe to say they retain alot of strength but not all, they're weaker but far from weak

I would say if they were 99 then, now they are 85-90. Rayleigh rep was enough for BB with 2 df to back down.

The arguement for WB at MF was he held back with his df due to his crew being in the way. Once they were clear he went nuts. An all out haki monster like Garp would cause crazy destruction

At this moment i doubt it considering the grwat feats blackbeard has achieved and garps age. But then again we must not understimate garp. Ages work a little diffrent in one piece. Blackbeard was afraid to fight with rayligh back on their encounter at amazon lily and its fair to assume garp is stronger than rayligh. Garp may come out stronger than blackbeard but in no way do I see him win their duel.