Is there anything in canon (preferably referring to comics Mxyzptlk) that speaks to the age of this 5th-dimensional being?
In Superman: Whatever Happened to the Man of Tomorrow?, we learn that Mxyzptlk is 6000(ish) years old.
Mister Mxyzptlk: The big problem with being immortal is filling your time. For example, I spent the first two thousand years of my existence doing absolutely nothing.
Mister Mxyzptlk: Eventually, simple inertia became tiresome, so I spent the next two thousand years being saintly and benign, doing only good deeds. When that novelty began to fade, I decided to try being mischievous.
Mister Mxyzptlk: Now, two thousand years later, I'm bored again. I need a change. Starting with your death, I shall spend the next two millennia being evil!
Probably impossible to state by virtue of his being a fifth dimensional imp.
The Fifth Dimension is an inter-dimensional reality that exists outside of the normally accepted space/time continuum. - https://dc.fandom.com/wiki/Fifth_Dimension
Since he comes from a place with with different rules for time, he could be any age.