Studies won't be done on long term risks in young adults taking it.
Do you want to take a drug that rewires your brain but has had exactly 0 studies done on what happens to users after 20 years? Alzeimer's drugs don't need 20 year follow up studies because, with the exception of early onset, the patients are, to put it bluntly, dead.
Actually right now Namenda is being used off-label for cognition enhancing properties, and suffers from the exact issue that its impacts on healthy adults is mostly unstudied. (FWIW I had negative cognitive side effects with it to the extent that I had to abandon it, but plenty of people seem to like it!)
If you want to talk even higher risk profiles, there are plenty of substances that may very well increase neural growth if given to children, but there is no way in hell any ethical medical team is going to ever run those studies.
For one thing, we know that plenty of awful diseases are the result of too much activity in the brain. No one wants to be the lead author on a study titled "Boosting IQ by 10 points in adolescents, with only a 20% risk of seizures."
If there's a drug that improves cognition, it will likely get approved for Alzheimer's.