The usage of the comma is correct. Now to explain the sentence:
"Retirees can start getting early, reduced benefits at age 62."
That is, retirees can begin receiving benefits at age 62 (hence, "early
" since 66 is the standard age at which they would receive the benefits); however, the disadvantage of receiving the benefits early is that they will get less money than if they would wait until they reached age 66. I hope that helps.