1)Will my age damage my chances at getting into a Russell Group uni, preferable Essex, Exeter or Nottingham?While - from what I've seen - those applying with Access who won't be twenty one upon entry can find it more difficult to secure places (specifically with RGs and high ranking universities), it's not impossible.
There are a couple of people in my class who are in that position who have all got offers from the University of Liverpool and I know of a particular TSRian who got into one of the universities you listed who was also in that position.
So, while stigma does exist, don't lose hope of getting into an RG altogether. Also, I think a lot of the stigma stems from those who have turned to Access after having dropped out of their A Levels - even then, though, the people who got offers from UoL had done exactly that, so it's not completely hopeless on that front, either.
2)I have a short course GCSE grade D in Statistics from when I was in year 9, do I have to disclose this considering I don't have the certificate?On UCAS, you'll have to declare it. I don't think there'd be too severe concequences if you didn't declare it to the college, but you might as well to cover all your bases (also, colleges are different and yours may be stricter on that front).
3)I did a trial week of sorts in college on a course I didn't want to do - a BTEC Extended Diploma in Computing - do I have to disclose this also?If you didn't achieve an official qualification, I don't
think you have to declare it. I'm not certain on that one, though.
4)Lastly, any advice on studying/preparing for the course? I plan on picking Psychology, Sociology, and Literature.Honestly, in terms of the course itself, there isn't much preparing you can do other than get A Level revision guides for all subjects. It depends on what you're applying to university for, but I'd strongly advise starting to read around your subject if you haven't already to have things to mention in your PS - you dive straight into UCAS when you start, so be prepared.
Also related to that, try and have a good idea of the course you want to do, your university preferences and a rough draft of your PS done before you start - I say this as someone who didn't do this and ended up very stressed for it.
I can tell you I do two of those subjects now and absolutely love them, so you should have a good time!