Not absolute number, but age distribution. One needs to have confidence that the country won't be facing major economic problems due to not enough young people to support an aging population - e.g. Japan.

GDP will tend to grow faster if your population is growing faster (in capitalist, developed economies). On a second-order (or maybe 3rd-order) basis, GDP per capita will tend to be faster if your demographics are skewed younger vs. older. Regardless, faster growth rates tend to result in higher real interest rates. Higher real interest rates attract foreign capital, which tends to cause currencies to appreciate.