When you're little, the only thing that matters to you is the toys. As a kid, when something came around that took your toys away, that was it. Younger siblings fell under that category of "something." Cue immediate ridiculous tantrums, complete with hitting, screaming, and the throwing of whatever toys were still in your possession. At least that's what it was like in my house, considering I had not one, but two smaller humans stealing my toys and my parents' attention. Sometimes, we would get along, and all the adults would go, "Aw, they're so cute, look how much they all love each other," until ten minutes later one of us pushed the other down a flight of stairs.
Then (if your age difference is large enough), you reach that age where you can appreciate how cute your little siblings are, even though you still think they're incredibly annoying. Your interests differ enough that you aren't fighting over toys anymore, but you're a teen or tween, so most of what you do is hate on the world. Your parents don't "get you" and your siblings are still too young to actually relate to. You try to teach them how to do stuff, and sometimes they get it, but sometimes they don't. It's really a toss-up. When they do get what you're trying to teach them, it's a small but important victory. When they don't, you freak out and vow to never try to teach them anything again because they're stupid (even though you know you will). And sometimes their antics entertain you and make you laugh, so you put up with them as a necessary nuisance for most of your teen years.
But then there comes a day where you look at your younger sibling(s) and realize they're real people too. Honestly, I don't think this is a moment any older sibling can forget. Even if you were always the best, nicest, most respectful older sibling, there's a moment when you look at your younger sibling and something shifts. The older sibling goggles come off and you see that your younger sibling, who you've watched grow up for as long as they've been alive, isn't only your baby sibling anymore.
I have two younger brothers, and for each of them, there was a different moment when this shift occurred. For one of my brothers, that shift came when he gave his Bar Mitzvah speech. He went up to the podium with so much confidence, and delivered the most heartfelt, kind, and beautiful speech I've actually ever heard. He delivered it with so much grace, poise, and composition that I was in shock. I cried, which is a big deal for me because I rarely cry.
For my other brother, that moment came my senior year of high school when I had just broken up with my boyfriend of nine months. My room and my brothers' rooms are somewhat connected, so any time I cried (which was a lot), my brothers could hear it. My crying spree had gone on for a few days (I know I'm not doing a good job of convincing you I rarely cry), and my brother came into my room. I was sitting on my bed, staring at him in silence. He walked over with a box of tissues and said, "It's gonna be okay," and then proceeded to tell me why this breakup was not the end of the world.
I think what causes this shift is respect. You've always looked at your younger sibling as someone a bit lesser (not necessarily in a bad way), but just as someone who needs guidance and can't navigate this crazy world on their own. Then, as if out of nowhere, they do something that you didn't teach them or tell them how to do, maybe even something you've never done before yourself. You can never look at them exactly the same again, because you've realized they've grown up. You've gained a new respect for them because now they're teaching you something. Compassion, confidence, whatever it is, it came from them and hit you right in the heart. And that's a moment no older sibling can forget.