Actually it is the rate of population growth that is declining, not the population.
According to the US Census Bureau: Iran's current population is about 72 million. Iran's rate of population growth will continue to slow until it stabilizes above 90 million by 2050. More than two-thirds of the population is under the age of 30. (
http://en.wikipedia.org/wiki/Iran#Demography)
So I don't think Iran is in any population problem (other than over population). Nor is the population considered aging. If anything the population is "under aged" which accounts for the increased disagreement with the conservative government.
As the old hardliners start dying off, the new generation will start filling the positions. Given enough time and no foreign interference Iran might just turn around by itself.
According to one of my professors (who is Iranian), other than a small radical population of college students and the hardliners, a large section of the population really does not have animosity toward the US. Like in the US, it is only the extremists that get the press it seems.
Strategically, this is not the time to attack Iran as it would only serve to codify the hardliner's policy in the minds of the citizens.
In the eight parliamentary elections since the revolution more and more moderates are being elected to office. Good changes are coming to the Republic... but slowly... but they are coming. In the amendment to the Iranian Constitution in 1989 even the religious requirements for the Supreme Leader were removed as the republic is readjusting the balance between religious and secular considerations.