From Jay R. Ritter, Founding dates for firms going public in the U.S. during 1975-2017 (updated January 2018
Users of this data should credit the source as the Field-Ritter dataset of company founding dates, as used in Laura C. Field and Jonathan Karpoff “Takeover Defenses of IPO Firms” in the October 2002 Journal of Finance Vol. 57. No. 5, pp. 1857-1889, and Tim Loughran and Jay R. Ritter, “Why Has IPO Underpricing Changed Over Time?” in the Autumn 2004 Financial Management Vol. 33, No. 3, pp. 5-37. Further details about the data can be found in Appendix A of the Loughran-Ritter article.