I've thought about Prevagen from time to time, but every time I research it, I back away from it. It's not from jellyfish, the main ingredient is synthetic. From what I've read, the only "studies" on the safety and effectiveness of the main ingredient has been by the company themselves, bias "study", no others I've seen to compare.
I take a lot of supplements, and I am concerned about dementia since there is Alzheimer's in my family. I take an Omega-3 supplement and D3, which I think are beneficial for the brain. Funny, when I was a young kid I used to go fishing with my father and he always told us kids, 'eat your fish, fish is brain food', that was way back in the 50s.
I think a lot of us couldn't recall a name or something even in our younger days, but we never thought much about it. Now that we're seniors, we really notice when things like that happen, and they worry us. I'm not sure any supplement can make a noticeable difference to be honest, but if I come across one, I'll definitely share it here. I've tried Ginko Biloba in the past, with no results.