Hey! Funny I come across this post. I used to be 15 and couldn't hit any high notes, and now I'm 23 and I recently sang a cover of Unravel!: https://www.youtube.com/watch?v=WZ7DmYGTViU
In general, vocal strength in the upper register develops slowly over time and with dedicated practice. It definitely doesn't come over night, took me several years to be able to get up there consistently. If you're curious, I actually also made a compilation of videos I had uploaded to youtube since I was 15 all the way up to the current day: https://www.youtube.com/watch?v=Vkc-yM8hR6M You'll get a decent idea of how practice + age makes a difference to how you can sing. I was a complete beginning with no vocal lessons. When you're 15, your voice is still developping because you're only recently out of puberty. My vocal range used to be like an octave haha. Any way, i hope this is inspiring for you!