Need help with my coming -of-age story

In my writing project, my main character grows from a teenager to an adult and I just need some help on how to make a smooth transition from kid to adult for him. I do not want to write him as an adult, but in a teenager's voice/pov/perspective. Yes, I'm scared of making that mistake. XD

So, any advice that you can give me?

A good book that shows somebody growing up (it's sort of a coming-of-age story, sort of) is I Am Apache. I'll summarize what the main transitions I noticed:

~ Attention wasn't drawn to the shift in PoV. You just went along with the story, and it's only looking back and actually tracking the character's voice that you saw shifts in the first place.

~ There were key events that allowed questioning from the MC. She started wondering who she was, and formed choices because of those events. But, as I mentioned, it wasn't reflected on in-depth. The story kept a forward progression.

Those are he two big ones. If you need more explaining, let me know!

This isn't necessarily my area of expertise, however when I write about a character growing up and maturing, I look to myself. For example, I'm currently 22 and a college graduate. How I react to things and responsibilities in life have changed since I was, say, 13. Where as once I would throw a hissy-fit for not getting something, or a boy not liking me back, or breaking up with me, I now react totally differently. So my advice to you is to look at how you have matured, and as you're righting, think of how your character would handle the current situation you're writing and see how they would handle it as the teenager in the beginning of your story, and an adult at the end. Hope that helps a little bit..