Ascians could be tempered, though. According to the main storyline Ascians exist primarily on a spiritual scale, not physical. If they are not physical, there's no reason to think they age. The 'people' we see aren't the Ascians: they are just possesed by them. Since tempering is the reclamation a Primal makes over a soul, this very well applies to Ascians.
There's also the possibility Ascian souls are, just like Echo users, claimed by another different entity that functions in the same way Hydaelyn does. This entity could be Bahamut or something else. Right now, there's no way of say if they wish to summon Bahamut because they are their servants, or because of some other scheme like just depleting Hydaelyn's aether. This seems to be the case, based on how they dedicate theirselves to push people into giving crystals to the beast-tribes.
There's also the possibility Ascian souls are, just like Echo users, claimed by another different entity that functions in the same way Hydaelyn does. This entity could be Bahamut or something else. Right now, there's no way of say if they wish to summon Bahamut because they are their servants, or because of some other scheme like just depleting Hydaelyn's aether. This seems to be the case, based on how they dedicate theirselves to push people into giving crystals to the beast-tribes.