> The most obvious solution to this is raising the age of retirement, but that's always politically difficult and somehow seen as cruel.

There is another problem: many people go into decline as they get older. My company retires some people at 40 with a full pension because most people physically cannot do the job anymore (I'm not sure if they are able to do much else after that). Airpline pilots are required to retire at 60 (I don't remember the exact age) because we are sure none are very far into mental decline even though a few could work for 30 more years.