How do you deal with being excluded at work?

I am 18 almost 19 and the youngest at work- everyone else is 21-28. Everyone is friendly enough towards me but I am always the last to know that someone ordered pizza and I always here about parties everyone goes to as a group but I never get invited. Everyone always tells me its because they older they have different social circles and has nothing to do with me. My best friend is probably 51 she is telling me this while we get our nails done together. All of my friends are like 38 and up. Can you say different social circles???? So why is it that I can make friends with anyone around my parents age but nobody my age!! I mean as far as being friends with older people we call/text, we go out for dinner, we get our nails done etc. and I can tell them anything! But I feel like such a misfit at work...