Well, it is true they say honesty is the ground rule for any kind of relationship. I mean if you think about it, how does a person feel when they realize you hid about your age; I'm not saying age is a big deal or not, but if that person was me, I would go like: "Okay if she/he hides a simple age, what else can she/he be hiding?".

Don't get me wrong, I'm just trying to simulate the other person's possible thought line. But friendships are built on trust and If a person rejects you simply because of a stupid age, then let it be.

Conclusion (and to answer your question): be honest from now on and you won't have to hide anything in the first place. And trust me, no ones in university or real life care's about your age, I mean that's just a number after all.