You're using input$choose_age, but choose_age isn't the name of your selectInput; it's the name of your uiOutput, that happens to contain your selectInput. The name of your actual selectInput is selected_age (so, input$selected_age).
I'd also strongly encourage you to clean up your indentation! It will help greatly the next time you forget a closing brace or whatever. You can easily do this in RStudio by doing Ctrl+a (select all), Ctrl+i (auto-indent).