Are you basing this off the manga translation? I'm not too sure whether they changed it from the novel or it is just a difference in translation. But from what I understand, the novel amrita makes you young for the duration of 1 year and wears off afterwards. Such that if you drink one bottle every year you could live forever.
Just confirming but did you read the Web Novel or Light Novel translation?
Because it's possible that there might have been changes made between the two adaptations since WN is raw format, while LN is polished.
And Manga always follows the LN as base.
In the Novel, by how much does the age reduction work?
I mean if it just says, "you would be young for 1 year", that's pretty vague. As if the supply isn't consistent there isn't really any benefit as it would just become a Drug Addiction case, as you would be dependent on the item to stay young. And in this case, it's possible to actually ban the item from selling.
I mean, no person would like to be reduced from 50 to 25 for just one year only to go back to 50 again if they don't have the money to buy the item.
I think the, "-1 current age" scenario is much better as the effect is longer and you don't suddenly change back to your old age. And even in this case it's possible to achieve immortality.