It can't hurt to look over stuff you've learnt once or twice, but don't worry about it too much. ^^^ good advice on the last comment. Start revising around Feb. Don't leave it too late so you have to cram all night the night before. It doesn't work trust me
I started revising when I was in year10 and if you use the right techniques your memory actually improves.. So you would be able to remember most things rather than having to revise them from the start. Don't do any hardcore revision just around one hour every day would help
I started around October the year before, did a few hours a few days a week, until February I've revised daily. Now my exams have started and I'm still cramming, but yes I'm glad I started early. I wouldn't recommend starting now, maybe just do notes for science etc. but don't actually revise them. I also wouldn't recommend past papers till at least january are you will run out fairly quickly.
If you're going to start revising in Year 10, the best thing to do is to make resources for Year 11. So put together revision notes to read through, make all your flashcards, so that you don't have to spend ages doing all of that when your exams roll around and instead you can just focus on learning from the resources