Legacy or not, I don't consider anyone really senior unless they have spent building and more importantly maintaining/fixing a system at least for 3-5 years at one single place and have done this at least 2-3 times across different companies. We all know that it is a lot easier to start a new/greenfield project with shiny tech etc. It is a lot harder to maintain especially once it is 2-3 years old and in production with tons of users.
The challenge there is that "legacy" codebase are just so much slower; you spend more time figuring out what needs to be done and how, than just writing code like in a greenfield project. And everyone knows the latter is just so much more fun.
