This is a bit of a challenging question for me.
I made up a lot of tunes on a synthesizer during my elementary years. One stuck out as something i thought was big, though I was much older, like 10 or older, that I played that felt like an intro to something big, though it was very simple sounding.
My first written song wasn't done until I was between 17 and 18 years old. It was designed to be a part of a then major videogame idea. I felt really good about it, and even my older brother felt impressed with just the intro chords.
My first recorded song was for an RPG maker game, and many people, even here on Newgrounds have heard it. I didn't record it until I was between 21 and 22 years old though. I'd been tinkering with GarageBand at Apple stores at least since my mid to late teen years, and finally decided to give it a go [productively] as an adult. I like the tune, and it does feel more finished than many of my other tunes.
I eventually did record the other tunes I had in my head or written down when I was younger, though I think very few sounded all that great. If anything the one I did around 10 or so in GarageBand has impressed me to the fullest, just rememering that I started with just one synth keyboard growing up.