You can guesstimate the average service life of a car using Little's Law: https://en.wikipedia.org/wiki/Little%27s_law

Share of new cars was 6.9% in 2019, ~5.0% in 2020, and scrappage rate was 5.1% in 2019 (source: https://news.ihsmarkit.com/prviewer/release_only/slug/bizwir...).

If the average in/out flow per year is ~5%, average life is 20 years. If 6%, ~17 years.