Psychedelia beat me to the rebuttal, but I'll post anyway to place extra emphasis. Marijuana doesn't erase existing memory. It inhibits ability to create new short term memory. Which is why things are hard to find. If your brain can't create memory to store where you placed your keys, well, you are out of luck. However, the fridge won't be hard to find as you know where that is even before you smoke. This effect is temporary. While lingering effects can stay for about a month, the majority of the effect goes away after a few hours, or after the high is over. The lingering effects depend on how heavy you use. After the high, however, the effect is negligible.
As far as the brain part goes, currently I can find no link between marijuana and loss of brain cells. Besides that, I can't be bothered to look it up, but I had been under the impression that recent research demonstrates that brain cells are actually regrown naturally by the human body.
On dementia, the cause and effect hasn't been proven yet. While a higher percentage of marijuana users are diagnosed with dementia, this doesn't prove anything. For all we know, the dementia is what caused them to smoke pot, rather than vice versa. Percentages don't necessarily prove cause and effect.
As for specific studies, since you didn't go to D.A.R.E I assume you must be great at independent research on this topic since you seem so confident. Here is one article, but you can find the rest. Minimal Long-Term Effects of Marijuana Use Found