You can pour old milk down the drain.
I have poured old sour milk down the drain before and it's never been a problem.
If you do pour old sour milk down the drain just turn the water on and allow the water to help wash the milk on down the drain so that it does not start smelling in the house.
I rarely have old sour milk unless I forget about it and leave it out of the refrigerator or I don't drink it all up in time or use it.
If it happens I just turn the water on in the sink and then pour it out into the sink.
You can also dump the old milk down the toilet especially if the sour milk has chunks in it.
The toilet is a good way to flush the old milk down the drain.
Or just pour the milk out or throw the old milk into the trash.