Are the cambridge masterclasses useful?

Not interested in applying to cambridge at all. I just want to know more about chemistry. Are they helpful in that respect?

I went to the last Chem masterclass, we did organic reactions by functional group level analysis and NMR (i'd never heard of the first before). They're great, you get to experience a day in the life of an undergrad more or less, which is always a good thing. Would definitely recommend going.

Would it be a problem that I'm in year 13? Considering its a year 12 event...

They dont really check much (They scan a ticket and wave you in), just say you're year 12 when registering and try to look a bit younger when you go. I would phone them to be sure, but I wouldnt have thought it a huge issue.
There is a bit on Cambridge admissions at the end (its 60-90 minutes long), just so you know.