People will complain about early onset dementia when they did a ton of weed and drinking in their youth. That's like standing infront of a train and

I haven't been hit ....and I did a lot of drinking and drugs when I was younger. I still drink and use cannabis, but a bit less than in my younger days
I know a number of people who developed early onset dementia, but none of them were doing a lot of weed (though some were heavy drinkers, others weren't).
It looks like the possibility of developing early onset dementia is more random, at least in ny experience, than having a strong connection to drinking and drug use.
I also haven't seen a lot of studies linking early onset dementia to drinking and weed.