I might suggest slightly condensing the original, modifying one date, and making two minor additions to fill out the citation. The original draft was 71 words (IEEE guidelines state "60-65 words are preferred for aesthetic reasons.") Tom Misa
In 1951 the Massachusetts Institute of Technology began developing an air defense system for the United States. The Whirlwind digital computer built at MIT became the centerpiece of the SAGE system. The MIT Lincoln Laboratory, working with partners in industry and government, carried out its initial development. By 1961 23 SAGE control centers were complete. SAGE was a forerunner of today’s digital computer networks. (64 words)
“In 1957, the first SAGE station became operational. By 1961, the system's 23 sectors were complete” <http://www.mitre.org/about/photo_archives/sage_photo.html> Original text stated original operation in 1958, but "what counts" for original operation might not be fully clear. Statement about the 23 centers being complete seems accurate and unambiguous.
“Lincoln Laboratory was established in 1951 to build the nation's first air defense system” <http://www.ll.mit.edu/about/History/history.html> Should the citation state, explicitly, that Lincoln Laboratory was created in 1951?