How old do you think Karasuma is now? Is he still an old man, or is he a younger man, did he possibly rejuvenate?
Karasuma Renya "Child of Seven" Theory
I think Karasuma Renya is in child-form like Conan, and is planned to be Conan's mirror-version on the bad guy's side.
"Nanatsu no ko", the crow song that the boss uses (File 475), can also be read as "child of seven", which would fit with that he chose that song as a theme reflecting his current state, apart from the crow theme (essentially that BO members are sending their text messages to a 7-year-old child, signaled by the dial melody).
Also, the fact that the research has lead to side effects such as shrinking (as seen with Conan, Haibara, Mary) and potentially halted aging (as seen with Vermouth) further presents the possibility that the boss was affected by them (Old man permanently stuck in a child's form).
It would also explain why Haibara described the drug that she was trying to create as "a foolish thing that most humans in the world wouldn't deem valuable" (File 384), because if the boss is working towards a cure for his state, then a drug that makes you age up fast wouldn’t be useful to anyone who can already age naturally, and people would normally value time and wouldn't want to lose it by growing old faster.
Haibara also monologued in Mystery Train (after recalling Elena's words about their dream research) "I didn't understand that this drug was something that shouldn't be made... but to prevent people from getting involved, depending on this drug is my only option" (File 821). This could have been a direct hint to that she was referring to the antidote drug as both the drug that shouldn't be made and the drug that she was gonna depend on to transform back to Sherry and sacrifice herself in order to prevent harm to her friends.
The APTX antidote effect being the true intended goal of the research makes the most sense, as it's not only consistent with both Haibara's drug description & Pisco's words ["who would have guessed you would develop it to this extent?" (File 242) | recreating the initial rare problem (shrinking) is a big step towards finding its solution], but that it also indirectly progresses the BO's research plans without Gosho having to introduce conviniently talented researchers to take over Sherry's research and to somehow progress the research in an even shorter time than the prodigy Sherry herself.
Instead, by having Sherry herself finish the BO's drug (the APTX antidote) but for Shinichi's sake instead of Renya's, all it would take is for the BO to steal it from Conan/Haibara so they can progress with their evil plans before getting defeated in the climax.
