There are many 20 years and older code bases currently. But if you start a new code base today it might get rewritten many times in the next few years because software users no longer have the power to stay on an old version due to the SaaS paradigm.
A lot of 6 month old code gets thrown away either because its been rewritten or because it didn't achieve its stated objective. Meanwhile a bunch of companies are relying on systems that were first created in the 90s because that software achieves its objectives and the cost justification for a rewrite isn't there.