If you go with the Terok Nor novels, then Dukat is round about 19 or 20 in 2318, when taking part in the first contact with Bajor, as he is a junior officer. That obviously makes him much older than the actor's real age by the time of the handover of the station. But I think it works if we reckon the average Cardassian lifespan to be a few decades longer than the average human one. Then, a man like Dukat would be in the years of his later middle age - still perfectly robust. I roughly read human 40s-50s for Cardassian 70s-80s.
I also reckon Cardassians might mature
quicker than humans, while being slower to age as well - Rugal looks very grown-up even though he's only meant to be about fifteen or so.
As for Tain, he possibly dies his hair out of vanity. And yes, I'd say Garak is definitely younger than Dukat, maybe by a decade and a bit.