@Kireato
Well,
@sssr already said the gist of what I was going to say, but let me expand a little in it:
Your point hinges on 5 assumptions:
1) All demons are born equal or close enough
2) All demons in general will have the same experiences in a certain ammount of time
3) All demons in general will have the same ammount of training in a certain ammount of time
4) The skills and spells of all demons are basically equivalent in terms of mana.
5) Mana is the only thing that rules the place in society of demons and the tasks they perform in it
If all of the above are true, you are right, there can't be any upstart demons and my point is moot, since older demon = better demon at all times. But ...
4) is definitely not true, from what it is said about Aura and her relationship between her spell and her mana. it is clearly stressed by Frieren that the synergy between her spell and her mana ammount makes her far more powerful than she would be with either of them alone and it is pointed out that the same spell in a weaker demon would actually be detrimental for that demon.
1), 2) or 3) are doubtful, because of ( like
@sssr pointed out ) Aura is said to be a unusual prodigy even among demons. So atleast one of those premises was not fullfilled by her ( either she was born superior or her training/ experience was superior ). Also , Gothic loli using the Dwarf axe technices pretty much invalidates 3)
5) is most likely not true as well, given the composition of the demon team sent to take down the barrier and invade the human world. Excluding the Greenhorn that tried to decapitate Frieren ( that was there clearly to perform some assasinations if needed, a task that probably doesn't requires much of mana , so a young expendable one will do anyway ), 2 of the demons there were probably not much older than 100 years ( Gothic loli and Blood guy ) given that they were seen as younglings when the hero party was around ... so they were not chosen for their age most likely.
In fact the main characteristic of the demon team is that, besides the Greenhorn, all of the team had contact with the hero party in the past and 2 of them actually survived combat with it ( Blood Guy and Aura ). There is no way that this was a acident ( given the pile of demon corpses we see in the past of the Blood guy, being a demon that survived a spar with the hero party was most likely not exactly common ) and most likely they were handpicked by whoever does that in the demon world and demon age apparently less important than experience with combat against humans ( and elves + dwarves ) in that choice. Given that so far this is the only glimpse we had of how demon society works , we pretty much have to assume this is typical ( no proof otherwise
) . Sure, it might not be, but we have no proof of that yet.
Given all of the above, the point you start, that demons mana only depends from living time and that it is the sole definer of their place in the demon society seems ... shaky at best.