Not that it was remarkably maturely, but I think that it's funny, so:

I'm maybe, I dunno, 5. The family goes to a theme park that has this ball pit. Little kids can swim around in them. I'm waiting in line and this group shows up sporadically, spread out over the line. Everyone in the group is somewhere in front of me in line except this one little girl, who is right behind me. They hit the maximum number of kids in the ball pool right as I go in, and the attendant says to the little girl "no, sorry, we can't have any more, you have to wait for the next batch" and she starts to cry. I say "let her go ahead, and I'll wait for the next batch". The ball pool attendant, who didn't want to deal with some hysterical little kid, says to me, "Thanks, kid. Here, you want to say something on the loudspeaker?" and without waiting a sec, sticks the microphone right in my face and keys it. I didn't, in fact, want to say anything on the loudspeaker, and say "No", which of course comes out hugely amplified over the ride's sound system. Every kid in the ball pool looks up petrified, trying to figure out what they just did wrong.

My brother very maturely didn't kill me when I when I forgot his Gameboy with Pokémon blue and six lvl 99 pokemon in the back of a taxi.

When I was 10, while riding my bike in the woods, I almost crashed into another cyclists who then swerved into a bush.
When I saw how he was laying there, I noticed he's in a thicket of burning nettles and his leg was bent the wrong way.
I told him I'm gonna get help, ran to the nearest SOS telephone and called an ambulance.
They had taught us in school what to say and to wait for further questions, which is what I did.
Then I ran back to the man and talked to him until the medics arrived.

When I was in secondary school I had to fill out a form over the summer to be allowed to leave early for some school activities. The form required a coach or teacher's signature. Since it was the summer, I had no contact with any faculty so I had my parents call the coach who told me to sign the form for him, which I did and I turned it in. Unfortunately, that coach left before the beginning of the year so when school started I was called to the principal's office and accused of forging his signature. I told the principal the coach told me to, so it wasn't a forgery, I was signing on his behalf, and I even put that on the form. He ended up calling the coach and he even confirmed but at that point he didn't want to back down and still denied the request.

After I left the two older ladies that ran the office found me and told me they just approved it after he left for the day and he would never know. They were super nice and told me they were proud of me for sticking up for myself. As a barely out of primary school kid it was really intimidating to be yelled at by an adult but I felt so vindicated when they recognized the inequity of the situation and helped me.

That principal ended up only lasting a year too.

In kindergarten after school I had a desinated place to wait for my older siblings to walk home with. they did not show one day (some miscommunications and conflicts in schedules). I walked several blocks to a payphone and called home. the eldest brother who I think was in high school was back by that time and picked me up and was very impressed.

Are you kidding? I didn't handle things maturely at 23! But hey! thanks for getting me to think about aaaaaallllllllllllllllll of my embarrassing childhood memories. Funny how there's so few good memories, but loads of embarrassing ones

I'll do a counter one: an event that I thought I handled maturely for most of my adult life, but came to realize recently that I really handled it immaturely. Warning: venting of implied child abuse.