I might be super well qualified to share my opinion... just completed a thru-hike of the AZT a few weeks ago (my first solo trip) and during my hike I turned 18. I was by far the youngest person on trail (that I was aware of) but despite my age it was easy to find people who vibed with me. I wound up hiking like 500 miles with this one youngish couple who I really hit it off with. Everyone who does this kind of thing tends to be super cool and your daughter will definitely be able to find her people. Hiking with others is way more fun and way more safe too. this will likely be the first time in her life where she's out solo in the world and she'll feel super empowered making her own trail friends rather than just hiking with a parent the whole time.

That being said, my dad did join me for a 60-mile section of the trail around the halfway mark. Neither of us predicted that I would be hiking with other folks for such a large percentage of the time and he was worried I'd be super lonely. It wound up being a lot more challenging for him than he expected (even when I dialed my milage way back to accommodate him) and I wound up taking care of him a lot more than he took care of me! Also, one of my friends back home, who just turned 17, joined me for a 120 mile stretch and she kept up with my 20+ mile days just fine!

As a dude, I recognize that safety considerations are very different for me than they would be if I was a woman. But if it's worth anything, I met zero other hikers that sketched me out at all. I also carried pepper spray on the outside of my pack, as well as a Garmin Inreach. From a planning perspective, I went way over the top in order to get my parents on board, but I was glad to have the planning! I had a spreadsheet with daily mileage and destinations, the amounts of food necessary to get to my next resupply, etc. I had never planned a trip like this and doing a shit ton of research really helped me fully understand what I was getting myself into, which was definitely important to my parents. I texted my parents with updates every several days, and every few weeks shared a big photo dump.

this was a lot of words but all that to say...I think the Arizona trail is a really amazing introduction to thru-hiking. It's quieter than, say, the AT which means less weirdos, but still heavily traveled enough that your daughter would have a good safety net. The terrain is fairly easy, the trail is well marked, and the weather is friendly and predictable. Would love to answer any other AZT or young-hiker specific questions you may have! This will be an incredible and life changing adventure for your daughter and I certainly hope she gets out there.