OP, there are times when it's hard to figure out who is at fault but this is not one of those times. Pay for his mirror and don't contact the insurance.

OP since you were in VA this law looks like it's relevant. Lots of people don't look before getting out of parked cars. It's a habit everyone should get into, especially when there might be bicyclists on the road.

Dooring Bill Is Now Law In Virginia
https://waba.org/blog/2016/04/dooring-bill-is-now-law-in-virginia/

You are at fault. This happened to my husband - but he was the driver and the vehicle that "hit" him was a delivery truck. Since the truck door was higher up than his sedan, it did damage to 3 parts of the body and was expensive to fix.

You are lucky it is only a mirror [well - your insurance is]

You’re at fault. He didn’t hit your car. You hit his car.

Did you practice due diligence by looking before opening your door into traffic?

Was the driver going at or below the speed limit?

Strict liability at play.

This is like the case where the dump truck was charged of involuntary manslaughter when he sideswiped a mom standing next to the minivan buckling in her child a few years back (granted there were other circumstances involved).

You are, and you're probably better off not going through your insurance for something like that.

He had the right of way, whether you were moving or not.

You are at fault.

You are.

It's my opinion you are at fault, but I don't know that is the legal answer. Regardless, I'd just handle the repairs with no insurance involvement at all.

OP, again, I think it is very kind of you to consider this young driver!

How on earth would it be his fault if he was just driving on the street?

You need to be sure the road is clear before you open a door into it. You are at fault.

I think you would be at fault...it is your responsibility to check for oncoming cars before opening your door.

You are; you should have looked before making an less-predictable move. He was making an entirely predictable move by driving the correct way down the street. He had no way of knowing a car door was going to open.

You are.

I was parallel parked on a residential street in Alexandria and another driver was passing as I opened my door. The edge of my door hit the edge of this passenger-side mirror and the mirror fell out of the frame and shattered. Would I be at fault or would he? The other driver is a teen and the damage to my door is barely visible, so if he would be at fault I would be fine skipping insurance and just each managing our own repair because I am sure his insurance would go up. If I would be at fault I would probably just go through insurance.