I'm in my 20s still, and I wouldn't make much progress on this at all without diagramming or leaving annotations on almost every line of code. Go ahead and do those things. If you don't, you could alternatively invest a hefty chunk of time into reading and absorbing the code so well you don't need to. And if you don't do that either, you're setting yourself up for failure in a way your managers probably protected you from when you were younger and less experienced.