I don't have my own teens, but I work with them and have read a lot about sleep and the effect of circadian rhythms on physical and mental health (Matt Walker, Satchjn Panda, Linda Geddes etc).
Yes, teenagers circadian clocks mean they're likely to fall asleep later and wake later than adults 30+, but later means up at 8am as opposed to 6am.
I would be very wary of allowing them to sleep in past 9am.
Disordered sleep has such a huge impact on anxiety, depression, stress etc I wouldn't let them lie too long. I'd be encouraging a sensible sleep routine as well as outside time (see below) and physical activity.
To have good sleep you need a large differential in light levels during the day. Bright in the morning/day time and dimming into the evening.
What feels like bright light indoors is orders of magnitude dimmer than an overcast day outside (I've tested it myself with light meters: ~400 lux indoors, even with huge windows on 3 sides, 5000-50000+ lux outdoors).
The dimming in the evening triggers the release of melatonin which causes sleepiness.
The other sleep trigger is the build up of adenosine (which is relieved when you have a nap). So people need to be awake long enough to build up this sleep pressure by being awake long enough before they try to sleep.
They won't be able to go to bed early after a late morning though. You'll have to turf them out of bed early one day.
Also, I know this is easy for me to say. Mine are yet to reach this stage!