I heard the sweetest thing from a colleague today. He is actuallay a very supportive colleague, as the majority of my colleagues are (there are non I would call "Unsupportive"). He heard I might find a job working with kids and he said, "Yes, that I could see. We'd rather have you here, but if you must go, it should be to children (he's Hungarian). There is so much child yet in you."
I think many of us send signals of being younger than we are. My colleague made a point that he didn't mean immature. maybe we are less defended in a way. Pooty poo, I think that your workmate's problems is that they are pretty blasted immature themselves and just can't deal with anyone different than themselves. I also got termed a Kíss ¤ss when I worked in the states because I was with the boss or manager just as I am with others, friendly. I have to have a reason not to be, and that reason has to be pretty major compared to others.
It is a pain having to deal with little girls playing at being grownups and having no idea what it's really like, huh? Where i work, there are some folks who aren't colleagues but share our department's personal room with theirs, and they are FIFTY+ and still acting just like that, little girls playing at being world wise adults. Best not to notice them. their Dumbness is irritating but it doesn't reflect on you. Vent away. We get it. HUGGSS