In my essay for Northwestern, I realized after I submitted it that it had a typo. Originally it said “a very early age,” but as I was cutting words to meet the word limit, I deleted “very.” Thats fine because it didn’t add much to the sentence anyway, but the problem is that I forgot to change “a” to “an,” so now it says “a early age.” It slipped past several people in proofreading, including me, and I only just realized it was there while reading it aloud to my mother. How much damage did I do? Did I blow it?
I highly doubt that such a mistake will have any major impact on your decision - the AO’s are looking for personality and character through the essay, it’s not some English grammar competition where you’ll be penalized for using the wrong article. Dont worry 
I would not worry either. My S19 had a similar typo on one of his apps and we were distraught for a couple of days before a wise friend of mine said “an AO would have to be a psychopath to keep that from admitting someone who they would otherwise admit”. That made us laugh and I also think it’s true.
Boy if that is your only glitch, you’re golden. I bet the web pages of each college has more typos than that.
Schools want the best students they can get. Why would they let silliness interfere with their goal?
The ‘clam fart’ thread from a few years ago comes to mind and the guy supposedly got into Yale.