Neloth appeared in both morrowind and dragonborn which takes place 200 years later. Miaq the Liar did too. Is it at all possible that Miaq is secretly an elf or does not age?

No, everybody knows that M'aiq is Tosh'Raka in his mortal guise. Or maybe Alkosh. Or Auriel.

There's no secretly about it. The Khajiit are an offshoot of mer.

That said, don't read too much into maiq.

M'aiq says his father and his father's father were also called M'aiq, and besides that he's just an easteregg.

"To understand the machinations of the M'aiq is to throw away your sanity in vain attempt."

- AWOL Ryan