Tarquin could be as low as level 15-16 or as high as Epic. More likely somewhere in between.

I have two quibbles with Tarquin being Epic level:
(1) It is impressive to toy with a group of opposing characters just 2 or 3 levels lower than you, because it can easily go wrong. If Tarquin is 21st+ level, it is just toying with outright inferiors. That is not impressive at all. It is just a DM Pet peeing on the PCs with impunity (yet again).

(2) I have suspension of disbelief difficulties with Epic NPCs lying around. The OotsVerse doesn't feel (to me) like a world where there are many challenges around to actually push past the mid-teens. IMHO, of course.

Obviously there is some significant degree of wiggle room on both points. But "wow, Tarquin must be Epic" is not really a positive point for how I would rate Tarquin, both as a character and the quality of writing for his character.

And the same points apply to Xykon, albeit to a much lesser degree. "Wow, Xykon is probably 27th level" makes me first and foremost think how amazingly incompetent he is, not how impressive or interesting his is.