I know that my little brother has been in college for about four and a half years now (he's double majoring...not just slow), but my sister-in-law just posted a picture of him grading art projects that were done by his students. I think this was the first time that I had a moment of realization that my baby brother is an adult. He has students that are actually learning things from him.
You have to appreciate that this is the first time that I've thought of him as an adult since he's actually going to be a dad at any moment. Apparently, that didn't trip any adult alarms in my head when he announced that months ago.
I have zero doubt that he's going to be an amazing dad/teacher/youth pastor/bike assembler (actually, I'm not sure if he still assembles bikes...but he has put together a few pretty quirky ones that would fit right in at an art museum).