No doubt Marilyn is correct about the lack of frequency for "since [indefinite measure of time] ago." Nevertheless, I'd like to call the readers' attention to the fact that "since many years ago" appears on several school and university websites, and others, as in the examples here:
"¢ We have identified the following as the five greatest problems: 1.The inadequate legal and administrative structure in order to guarantee the fulfillment of the fundamental policy on the Galapagos that Ecuador has formulated since many years ago and that the President of the Republic has recently made a comment on while addressing the National Congress, which states that the ". . . Galapagos is a region destined to conservation, and therefore, any activity must fall, and be conditioned within this principle . . . "
"¢ Discrimination has existed since many years ago, for example, in 1845 when Texas annexed to the United States and became a state, Mexican Americans were discouraged to fulfill their right to vote by poll taxes, literacy tests, and old-fashioned intimidations. Hispanic Americans lost
"¢ The ten leading pharmaceutical products in 2002 were all branded products, eight of which were products of multinational companies; eight have been off-patent since many years ago and six have generic equivalents in the market;
"¢ Since many years ago, the role of higher education in Uruguay has been emphasized as strategic for the future of our country.