Paul Krugman comments that Portugal can be a situation where its aging population, combined with a large outflow of younger people due to high unemployment, can lead to an ever worsening financial situation where fewer workers are left to support a larger debt and non-working population. (Note, the key factor here is the migration, not the aging.)

That pretty well describes the picture with Puerto Rico, with a large segment of its working age population moving to the mainland United States, leaving the island with relatively few working people to provide taxable income. The upside for Puerto Rico is that at least it has benefits like Social Security and Medicare covered by the national government, compared with Portugal, which must pay for its equivalents from its own tax revenue.