Its never too early. It's probably one of the hardest things to complete prior to applying to medical school. The challenge is, completion is not adequate. You want to make a GOOD personal statement, just just A personal statement.
One tip I'd recommend, is freewriting. Trying to make a cohesive, complete statement in one pass can be overwhelming. In your freewrites, you'll find some good pearls to include in your personal statement, or you may find them useful to save for secondaries. I felt that I had more creative, emotional, and genuine perspective when I freewrote compared to when I was writing simply to answer the prompt.