Startup firms created all net jobs during most years since 19771. Existing firms create jobs, but they also slash more jobs than they create every year. Net job creation relies on young dynamic startups.
Here is the iconic chart Kauffman published in 2010.
Haltiwanger, John, Ron S. Jarmin, and Javier Miranda, 2008, “Business Formation and Dynamics by Business Age: Results from the New Business Dynamics Statistics,” CES preliminary paper. ↑