Your birth rate is probably more affected by the number of 18-40 year olds than by your health index.
If I was in your situation (and probably will within the next game century), I'd probably cut my health index down to around 100 - 120, live with the unemployment (and leave the nurses, doctors, and sr doctors as unemployed if at all possible), leave it like that for about a rl week or so (about 3 or 4 game years), and see what effect it's having on my population. I'm surmising that it should kill off a large part of the older population, and at the same time, increase the proportion of breeding age (and working) people to total population. At that point, I'd probably bring the health index back up.
While this is all theoretical to me, I can't think of any other way of handling the situation you find yourself in. If others have ideas (especially if they have practical implementation experience), I'm also interested in hearing their ideas, but lacking any other input, this is the route I'll probably go.