64GB should be more than enough for anyone, unless he or she plans to keep 64 or more GBs of apps, photos, documents, videos, and music 'on his or her device'. Still, it depends on the person. Being a teen is irrelevant in this regard.
Most teens take a lot of videos and pictures; have him/her install Microsoft OneDrive app and have it setup to backup photos and videos when Wi-Fi is on. That way they won't keep everything ON the device, and 64GB will be enough. It should be more than enough anyway, but just in case! Teens also like a lot of apps and games, so it'll work out.
I have 4k photos and videos, 130 some apps and I still have 23gb left. I also have a 64gb iPhone.
Just know that unless one has a 16GB device, and is adept at using that amount of space - 16GB will not be enough... - I'd go 64. Will hold resale value better than lesser capacity devices.
In addition to the OneDrive app, they can also use other cloud based apps like Box and Dropbox for photos, videos and gifs. Using these cloud based apps frees up space for the music they will want tons of. 64 should be plenty.