1. Obviously, it's a mess for pay-as-you-go social security as you have fewer workers supporting more elderly.
2. Older population means less innovation. Old people are more stubborn and hang on to outdated worldviews (there are exceptions, of course, e.g. Pres. Nelson).
3. Fewer minds on our most pressing problems, including environmental ones.
4. Fewer geniuses whose ideas benefit the entire population at no cost. E.g. the proportion of math geniuses in a population is constant.
5. Smaller market size means less incentives for entrepreneurship.