Is it okay to text a 28-year old when I'm 18?

I met a 28 year old on my college campus. As a freshman and going to college it seems as though everybody is around 18-20 years old. Well apparently not after meeting several people. The 28-year old I met I actually thought was a 19 year old. Before finding that out, I joked around like a silly kid and acted my age because I thought he was similarly young too. After finding out his age and still talking to him like a buddie, I feel as though I'm texting a teacher since he's 28!! It doesn't feel the same around him anymore now I know his real age. And its actually around any older adults I feel awkward towards them as if I cannot be silly and be myself. I feel like I have to be mature since they're older. Why do I feel like this? Is it normal