I notice the same thing at age 40. As a former state spelling bee champion, I could spell a estimated 50K-70K words (larger than the vocabulary of most college grads), so I doubt that this was due to the increasing size of my vocabulary.
I first noticed the problem when I began to use spell-check on my word processor (which I’d stubbornly refused to do until recently). I still have no difficulty spelling words in regimes where I don’t have a spellchecker, such as foreign languages, ancient languages, medical terms <where I don’t dare let a machine change my words: ileus, ilius, ilium, etc. are all legitimate words, but mean very different things; in nerves and arteries ‘median’ and ‘medial’ may both exist, half a body apart.>
That doesn’t mean I always type them correctly (my fingers have minds of their own, and are always trying to anticipate the word I’m trying to type. Moreover they are incredibly clumsy), but I do catch the errors in hard-copy proofreading. (I’m not as good at on-screen proofreading - but that’s probably partly because I only print out the important stuff, where I’m going to pay attention, while e-mail, posts, and other casual typing I care less about gets a cursory on-screen once-over, at best)
I am rather more disturbed that I find myself making grammatical typos (there vs. their; it’s vs its) which I never made before -but only when I work on computers Interestingly, this tends to increase if my wrists or hands are stiff due to (e.g.) injury ir cold. As I said, they have minds of their own.