I just completed my first year and it was really pretty tough, felt completely lost most of the time, had no idea where to go and what events were going on, and due to my anxiety didn't know how to approach people with the hope of hanging out with them or becoming friends, so the result was I never made any friends and I had a lot of difficulty with my studies also. I have no idea how I messed it up so badly because it seems like everyone else settled in fine, made plenty of friends, were getting on ok with their studies as far as I could tell and still had time to join societies and go out nearly every night. I did go to uni though with mental health issues that have never been addressed and caused me a lot of problems (anxiety and depression) so that made things very difficult, I did try and get help with it at first by using the counselling service, but I found the counsellor to be really terrible (to the point where she really shouldn't be qualified to be one) that I never bothered to go back, and being to embarrassed to talk to other students or a lecturer about it I didn't know where else to go, and just hoped things would get better.
So yeah depression certainly makes things much more harder at uni, and uni is hard to begin with, but I still wouldn't let mental health issues put you off from doing what you want to do. I mean I didn't want to be depressed and I wanted to go to uni to learn about a subject I am interested in and to make new friends and I still want to do that. Obviously depression makes it much harder, but you need to fight it. Also I wouldn't let age hold you back, 20 is young and it is not abnormal to go to uni at that age, I started at 22 and I don't consider myself 'too old', so its really not something worth worrying about you should go to uni if its what you want to do and not be concerned about your age.
As for giving uni a miss for the year, I would only do that If you can actively get help and work on whatever it is that is bothering you now, otherwise it would probably just be a waste of your time. If you could find out what course texts you need and what it is you need to learn in your first year though, then taking the time out now to start your studies early before diving into it might be really helpful.
Alternatively if you want to start, then maybe you could contact your uni now before starting to see if there is any extra support they can offer you which could help you settle in better and avoid feeling lost. I think most universities offer some peer support system to help students feeling apprehensive to better settle in, I never bothered with that, feeling a bit to embarrassed about it, but I was feeling very apprehensive about starting and I did feel very lost, unsure how to talk to people and also had difficulties with studies also, so maybe It would have been worth it, and so might be something worth considering for you.