Hi there,

I have looked for it for ages, I can not find it anymore.

As I recall it is a yaoi manga oneshot in historical traditional Asian set. Color setting monochrome.

I do not recall much of the content, only that there were two men having sex which another, and one of them keeps getting younger from like mature to 18 or something.

I have no idea what the title is and whatsoover. Only the people were wearing asian traditional clothes (probably kimonos).

Thanks for the help.

You could try looking at this list
https://www.mangaupdates.com/series.html?genre=Historical_Yaoi&filter=oneshots&perpage=50
I'm sorry I don't know which one it is.

Is it a doujin? Because I know a doujin that fits the description except the one getting younger starts out middle aged then turns into a teenager, then a child.

Is it Hakobiya by Harada?

[Harada] Gintama – Wakagaeri Hen

I managed to find it through your assistances!