Which part of my answer gave the idea that this is about age? FWIW, academia have never been good at teaching these skills, so the answer to your question is "

Which part of my answer gave the idea that this is about age? FWIW, academia have never been good at teaching these skills, so the answer to your question is "You'll stop being junior with regards to refactoring complex code bases whenever you have enough experience of refactoring in big old code bases". Mentors pushing you in the right direction can also help. But this is very much a skill acquired by actually doing.