FaNan

Their ages were never specified in the story so these are just my headcanons.

Regarding Houji's age, since he is older than Arima (29) and younger than Shinohara (37), I think he is somewhere between 31 and 35 when he first appeared in the og Tokyo Ghoul. I mostly headcanon him as 33.

For Tatara, it's more vague and it's mostly based on "how young do you think he was /when Yan was killed by Houji"

Look at this illust, guess his age, and add 5 to it, you will get Tatara's likely age at the start of Tokyo Ghoul. A clearer explanation (just my own speculation) can be found here. And yeah he was 18 at most when Yan died (18 is a generous number) because Tatara was not yet an adult when Houji first came to China, as implied by Ishida Sui in zakki:re.