It's never too early to volunteer! In fact, that's how I got my job as a PT Aide. Can you imagine how many hours you'd rack in while you're getting paid? And the experience! Even if you're not getting paid, it's a pretty fun volunteering job.
Anyways, I'd recommend you try volunteering as soon as possible to see if it's the right fit for you. If you love it or hate it, at least you know right away instead of spending a bunch of units taking classes you don't need.
To start, just give them a call or visit them in person. Ask them about shadowing/volunteering; that you're interested in becoming a physical therapist. Try a few different PT clinics. It took about 4 calls before one called back for me 🙂 Good luck!