For me, it started since Uni. I didn't feel like I was 100% ready then of course, but after looking into the freelancing market, I realized that the sooner I make a start, the easier it would be to sell myself to clients when I enter the big world, since I'd already have a few feedbacks and some work for my portfolio, and a few people I could ask for a reference. I just didn't chase very high value projects straight away, but started from completing some simple graphics related work that I knew I had the skill to do. A few simple 3D Models rendered, Simple video compositing tasks that people needed and things like that. So even though I didn't make huge bucks at that point, I started building a bit of my own network and had a few things to demonstrate my skills to future clients. So by the time I graduated, it was already much easier to attract a few clients and start making a living.

So I would say, just start early, but know your limits. Start doing stuff that you know you can complete, even if money isn't big, and build professional relationships with at least a few people before you dive deep into the freelancing career. By that time, you'll know if you're really ready for the big dive or not.