ASAP!!! It's NEVER too early to start. 13 is a great age, because:
- You're (I hope) not trying to make a living, just get some extra cash. This takes off a lot of the pressure
- Compound interest works best when you start young, and there's no reason you can't save some of your earnings
- You're too young (at least in the US) to take a normal job, so you'll have to learn how to generate income in other ways <- personal bias alert
I'd start with what's interesting to you. If you have a topic you think you could blog on and other people would like to read it, then start a blog.
If you like to make things (woodcraft, clay modeling, knitting) then sell them on ebay or set up a website.
If you like to bake/cook, see about setting up a regular bake sale or helping busy families cook dinner.
If you like kids, try babysitting. If you like outdoors, try lawnmowing (to make your business stand out, offer general landscaping like raking leaves, weeding gardens, etc.)
I'd be happy to help you brainstorm income generation. Private Message me with your interests, hobbies, or things that you don't
currently do but are interested in trying.