Access to historical SEC filings varies by database.
Bloomberg - Generally speaking goes back to the mid-1990's; however, some companies go back as far as 1990.
EDGAR - 1994 to Present
Lexis-Nexis - Full text EDGAR filings from 1991 to present. There are limited 10-K, 10-Q, and proxy filings from 1987.
ThomsonOne - Some companies have filings as far back as 1968. Some are digitized while others require you contact ThomsonOne to get a copy.