The most telling stat is average age. Depending on its level, it becomes increasingly difficult to maintain natural population growth. In short, the lower it is, the more growth you have.
Your average age is currently 34.96. An average age of about 34.5 is the break even point of 120-130 health index. Less than 34.5 and you get growth, more and you see a population decline. The downside of more health index is that the average age will continue to go up and up and always leads to population decline, eventually.
The higher average age also leads to a larger older population, which results in a smaller workforce relative to your population. Note that if you try to keep adding hospitals for higher and higher health index, you have the larger problem of a smaller workforce and then employing a larger proportion of those workers in your health system.
If you choose not to raid, you have a long road to correct this on your own. I would continue to advise not to purchase population, as they would mimic your current demographic. It is best to work on reducing average age, possibly with a somewhat lower health index for some time. Perhaps lowering health to 110, or even 100,finding the bottom for average age for that health level, and then purchasing pop to get things going in the right direction again.