Economists are warning that Europe's aging population could drive a slowdown in growth on the continent.Janina Steinmetz/Getty ImagesThe global population is getting older. The trend will lead to lower returns, earnings growth, and valuations, JPMorgan strategists say.If the share of people over 65 increases 1%, returns will decline 0.92%, they estimate.The aging of the world population is expected to accelerate in the coming decades, which will likely produce bad outcomes for the stock market, JPMorgan strategists say.The strategists say that historically, an older population has led to declining returns, lower earnings growth, and weaker valuations.Over a 10-year period, a 1% increase in the number of people over 65 is correlated with a 0.92% decline in annual stock returns, the strategists calculated in a report released last week.They point mainly to slower growth and falling valuations.With an older investor base looking to save for retirement, investment capital declines, leading to less innovation.