Moving through 30s, mental deterioration

I'm 33, and my brain has been deteriorating for years now at a frankly alarming rate. My memory especially. I have difficulty juggling multiple ideas at once, performing calculations in my head, remembering names, keeping a mental to-do list, things like that. It's even affecting my speech, as I have difficulty thinking of the right words to say, or storing what I want to say when it's my turn to talk. (To be clear, none of these were problems in my 20s.)

I expected this would happen eventually, just not this soon. Maybe when I was 55 or 60 or something. So, is it normal that it's happening so soon? Do other 30-somethings have similar experiences?