That’s gold, Jersey, GOLD!!
I do homophone substitution quite a lot recently; one example I’ve mentioned before (and which happened while posting at SDMB) was the substitution of “value” for “vowel.” (Not sure if I’d make such errors hand-writing as I do very little of that nowadays.) I’m approaching late middle-age. (How old are you Asimovian ? 
)
I also omit words, etc. I find most of these errors on Preview before clicking Submit. Unfortunately, the frequent cases where my sentences are grammatical and sensible, but confused because they rely on unclear connections, don’t show up for me on Preview. :smack:
35, but I have the mentality of a 90-year-old. Does that help?
Oddly enough, I didn’t start typing there for their until I got to this messageboard. I’ve thought so poorly of people who did that, and I never realized how easy it is to do.
But my biggest problem now is leaving out words. I’ll think the word, get distracted by something (usually how I’m going to write the rest of the sentence), and think I’ve typed it when I haven’t.
BTW, my brain just did something weird here. I typed “problems” instead of problem, so I also automatically typed “are” instead of “is.” Verbs apparently have to agree even with my mistakes. 
When I’m typing I have a bizarre tendency to sometimes conjugate verbs basically at random. My favourite trick is to use the suffix “-ing” instead of whatever is appropriate for the tense that I want to use. I hate it: it makes look like an illiterate idiot.
I see what you did there… 
With your excellent typing proficiency, you have noticed that mere practice has little to do with it. What you have described relates to the dynamics of the conscious mind and the “sub-conscious” mind.
Some cases of this are called ‘muscle memory’. It feels like your fingers have decided type some other word you may use more often. I’m sure some very complicated brain process is involved in typing, whether composing text or simply copying it. If your thoughts are running ahead of your typing, it’s going to happen a lot.
I do all the things mentioned upthread, and another really weird one.
Certain words I mistype every single time I type them, then have to go back and correct. It’s muscle memory of an error or something. The only one I can think of right now is that I almost always type:
actualy
Instead of “actual”.
I know that actually is spelled with two Ls, so I have no idea why my fingers always do this. Nor can I think of any other word that ends with “ualy” or “tualy” that my fingers might be emulating. It’s very strange.
My fingers also tend to spell out phonetic translations of what my brain presents. I’ve been attributing it to cognitive side effects from a medicine that I take. Maybe it’s just old age creeping up on me after all.