Anyone can go, we don't interrogate them as to what year they're in. Usually Y11, Y12 and Y13. Some Y13s will have already have submitted their forms (first UCAS deadline is 15th October for Oxbridge, vet med, medicine and dentistry).
You have to apply for tickets for some Open Days. I know people have said you can just turn up anyway, but my DC said when they were helping at a recent Open Day prospective students had tickets with barcodes so it wasn't as easy to slip in ticketless as previously.