There really is no such thing as a "Fascist Manifesto".

I have briefly - albeit not concisely - examined the formulation, composition and background of a few outlines of a program for the 1919 fasci di combattimento in a previous answer that I am going to reproduce below.

As to the specific point you raise. It's worth noting that no version that I know of called for the lowering "of the retirement age of Italian citizens". The first published version I am aware of included a line calling for:

a necessary modification of the proposed law project of insurance for invalidity and seniority lowering the age limit from the one currently set at 65 years to 55 years.

It is, as the text itself made clear, a proposed adjustment to a law project destined to go under examination in a nearby future. And, given the imminent end of the legislature, and the severe unsteadiness of Orlando's Ministry, there was little reason to expect the project to actually move forward without significant revision and a substantial delay.

The other - more frequently cited - version of the "program" replaced the line with the following:

a modification of the proposed law project of insurance for invalidity and seniority setting an age limit according to the workload of each profession.

As to why those lines were included, it has very little to do with the specifics of the work insurance legislation, and more to do with the general context of its composition. Which is examined in the answer below.