Why would carbon dating be relevant for extra-terrestrials? Without knowing the amount of C14 in their native environment, I don't see that you have a basis on which to calculate age from the current amount of C14.
They rely on a few assumptions that seems universal to living organism on earth. One basic one is if that org live long enough on earth, their body composition of c14 should roughly match earth. This "long enough timing" can be short as few months to years depending on which part of the body you sample the dna for analysis. Is it definitive no. It is even accurate enough to be admissible in court very similar to lie detector. But the science and statistics analysis around it are very solid and well develop that for some people it is good enough for a guesstimate than just mere gut estimate. Of course if those aliens travel to earth from their home world and immediately die, then the c14 ratio will be off. But then again given so many things we don't know, some trade-off and assumptions needed. We revere Newton even though his formula is way way wrong when we discovered Eibstein relativity. Yet, we all still use Newton good enough formula in our day to day physics. Same with C14 even if it is used for this ETs.
Or DNA, for that matter, why would they come close to having DNA let alone sharing any %age with terrestrial species?
it may be that evolved humans time-travelled back
and it may be that they travelled from a branch universe, or our own