I am guessing some comments have been erased, as I am not seeing anything all that rude
I think it is normal and fine to be worried. It is what moms do
I agree with others that for an international flight it is unlikely that you will be allowed to accompany a 16 year old to the gate. I also agree she will likely do just fine. (I flew alone from Denver to Madrid, changing planes in Newark at 15. I returned at 16 taking a train from Valencia to Madird, spending a night alone in an airport hotel and then flying through Chicago on the return. I did just fine. I don't think I was more aware/mature/what have you than most kids that age. It is really not that big of a deal
).
My own kids have not flown internationally alone before (though DS12 will go to London from Germany on his own this summer and DD14 may go back to the US this summer and will for sure next summer, and she is looking at an exchange program that will hopefully send her to Indonesia or Brazil--those are her top two choices--which would have her flying alone there in the next year or two).
I can, however, share the story of them flying over Christmas break in the hopes that it will make you feel better:
The kids were together, but younger than yous having turned 12 and 14 the week before.
They stayed a few extra days with their grandparents and were to fly from Denver to Boston alone. The grandparents had planned on getting them into the airport, checked in to their flights and walking them up to security (but not on getting passes to escort them). My mother in law ended up throwing her back out, quite severely, the day before their flight. With the in laws at the hospital, my sister in law dropped them off at the curb on her way to work that morning. They did FINE getting themselves checked in, checking their luggage, finding security, finding their gate and not missing getting on the flight (even though they had tons of time as sister in law had to get on to work and left them about an hour earlier than they needed. They had breakfast at the airport and just watched the time).
On the other end, the friend picking them up in Boston (I was flying in from elsewhere later that day) got stuck in traffic after a bad wreck. The kids did not panic, got their luggage and waited there.
Kids are really pretty capable of this stuff when they need to be (oh, and yes I worried about them until I was with them again--though I did not know about the back out/dropped at the curb thing until I had then--thank goodness or I would have worried more
).
...or when I'm in a movie, she calls and leaves a message and because I'm in said movie I can't call her back for 2 hours! 
