Steady state population should be the long term goal, at least until we start moving into orbital colonies or something. But unfortunately that's an anathema to growth oriented economies which makes it the bad guy in economics and politics.
The population only went over 2 billion in 1927. Dropping from current numbers is fine, and less competition over space would be nice.
Not to mention the changing political dynamics of having a lot of old people outnumber younger people by 2 or 3 times in the voting booth. The gerontocracy is bad enough now. Imagine it 3x worse.
Looks fine to me. What's your definition of "old" here? Are you specifically worried about Korea like you mentioned in another post?
I'd worry about the implications of a global 1.6 birthrate if they were actually going to happen. I don't think that's very likely, though. I don't know if we're even going to get down to 2.
Unless someone thinks of a democratic, scalable solution it's going to be a lot of pain for a lot of people for a long time.
A rapidly shrinking population is not ideal.