Age gap

I'm 21 and the PA I work for is 45 and we have recently started messing around. Haven't really done much yet. Mainly kiss and flirt at work. Is this wrong? At first I felt this knot in my stomach but his I'm getting used to it. I am thinking it's because he's so much older than me. He could be my dad literally. Is this morally wrong? We both expect nothing to come of this. It's just pure fun and something to do when we're bored and need to make the time go by at work. But I still have this little feeling that it's wrong. Someone tell me this is normal.