As both a woman and a manager, I have a LOT of concerns about how your manager is handling this situation. They shouldn't be warning a teenager to stay away from a(n alleged) sexual predator, they should fire the man if actual sexual misconduct has occurred. If there's no proof or even allegations of misconduct other than the relationship age gap, then they should never have said anything to you. By doing so, they are - intentionally or not - trying to make you responsible in the event something happens; i.e., "I warned her to stay away, but she didn't listen."
You need to escalate this, because it's NOT your responsibility as a student in a work placement role to handle any part of it. If you have access to your manager's manager or some other person in a position of authority at the farm, and are comfortable doing so, speak with that person. (@GeoffreyBrent makes a good point in their comment below, which is that there's a real possibility the farm's upper management may be part of the problem. You know the situation and the people involved better than us Internet randos, so use your best judgement here. If in doubt, skip this step and go straight to your university.)
Explain what your manager told you and see what the grandmanager says. If they're remotely competent, they'll deal with the situation as appropriate - first and foremost by ensuring your safety regardless of the truth of your manager's allegations. Without knowing whether there's proof that the man is any kind of sexual predator, it's hard for anyone on this site to say exactly what should be done. But roughly speaking, if there is proof of misconduct, the man should be fired; if not, your grandmanager needs to address your manager's allegations with your manager. (As commentors have mentioned, since a 17-year-old is of legal age in Australia, calling the employee a "predator" solely because of an age gap could get your manager in legal trouble. But again, that's not your problem; the most you can and should do is make your employer and/or your university aware of the allegations.)
If you're not able to escalate to someone at the farm, then escalate to your university. I totally understand that getting a work placement is important, but it's NOT worth the risk of being sexually assaulted (or getting drawn into some weird feud between your manager and the other employee, or whatever's going on here). Make noise until your university either gets your employer to deal with the situation as above, or gets you a safe placement.
Again, whatever is going on here is NOT your responsibility. While you should always be mindful of your own safety, no manager in their right mind would ever tell a teenager to "stay away from" a potentially dangerous coworker and then wash their hands of the situation. Escalate this with your employer or your university (or both) until you feel safe, either at your current work placement or a new one.