There are four cognitive symptom categories for Alzheimer's disease: aphasia, amnesia, apraxia, agnosia. These are broad categories that our brains handle somewhat separately. People frequently do sustain a brain injury to the left side of the head that damages his or her ability to process language and speech without affecting any other cognitive abilities. Our ability to utilize language is somewhat separate in the brain from areas responsible for the ability to reason, remember or use muscles.
So transient forgetting of spellings of common words would be processed in the language areas of the brain, and deficits of that ability are called Aphasia.
Forgetting daily chores falls into the category of Amnesia and is handled by a different area of the brain.
But your examples are about as non serious as they can get. Everyone has minor lapses in cognitive ability. So I'm hesitant to suggest seeing a doctor as I don't want to be blamed for a bunch of people getting laughed at when they go see their doctor for symptoms that lack of sleep may cause. Pose some more serious examples or extensive repeated lapses of forgetfulness or difficulty with words, or motor control, and I would have no problem suggesting you speak with a doctor.
Yes, Alzheimer's disease can happen to people in their late teens. But it is very rare. Most commonly onset of this condition is 40 or above.
And I simply don't know enough about aphasia research to answer your third question. I suspect the answer may be out of scope for this forum, but I don't know. Perhaps someone else will. However I must also caution you that dementia is one of the most serious, but isn't the only cause of cognitive lapses.