Im 23 and had never been on anti-depressants or SSRIs..nothing like that, until one month ago when my doctor put me on 20mg citalopram. It was a last resort decision and I had a lot of reservations about it.
But I was pleasantly suprised and it has given me a real boost which has meant the things I wanted to do but could never manage before, I have now been able to get on with. I realised I was making my life more difficult that it had to be and had given up on tasks that were actually very simple but needed a bit of enthusiasm. I have the occasional ever so mild low feeling for a few minutes sometimes now and then but it is nowhere near like how it was before. I'm on my second box now, so month 2 and so far so good.
I'd say to anyone who isn't noticing a change, give it time and if you have any bad side effects at all by all means go back to your doctor and try something else that you are compatible with. I guess it's like trying to find the perfect partner, hang on in there and you'll find the right one for you :p