Honestly, since this is a medication that would affect cell/body degradation and not growth, then it would make sense to introduce it at birth! This is the same aging system that i'm using for vampires in my world (except cell degradtion is halted completely, allowing biologic immortality), so in my world even if a child was turned into a vampire they would continue to age normally until reaching a visual age of early-mid twenties where it would stop completely. If an old man is turned into a vampire though (or starts taking the medication for the first time such as in your world) then he wouldn't magically become younger, his body would just stop any and all future degradation. Though i am unsure yet as to whether it would reverse/improve degenerative diseases like parkinsons or alzheimers/dementia. My first instinct would be yes, because we know that cells,