My friends think I'm the same age as them, but I'm a year younger.
Do you think they'll ever find out your true age?
They might have found out a long time ago, but forgot. They treat me nice and make me feel comfortable, but them not knowing makes me uneasy.
You are probably better off being honest with your friends about your age. They may feel that they can not trust you if they find out that you have been lying about something as minuscule as your age.
I don't want to risk that they will ditch me.
Being honest is normally the best policy in most cases. In this one, being honest shouldn't be all too difficult as being a year younger than your friends is not really a big deal.