Nobody was able to maintain my first app out of college in 1969 during its 18 year run. I wrote my own memory reuse routines, designed my own relational database, built my own functions (now known as methods), and solved 40 equations with 90 unknowns. It increased the company’s share of the total market by 20% (not just 20% of what it had).
I learned the most in my career with my second, relatively easy, business app. It was designed at a senior level but coded at junior or mid-level. Looking back, I would rate myself a senior developer, with less than 2 years experience, at the end of that app.
Here is an example: