I know what Pigeon means, I can see how it could make you feel not as close to someone.
I remember my older cousin getting married when I was younger and we were all invited as a family, so I have no idea how I would have felt if excluded.
Now my two sons who are teenagers really look up to their cool older cousin. They have joked for years about when he gets married who will ride in the limo with him, etc. If he didn't invite them, I am sure it will feel like a slap in the face. I wouldn't imagine a life-long rift over it, but the message would be there for them to read. Humans are social, thinking creatures not robots. Sometimes you are hurting people more than you know.
Also, even if it is a "child-free" wedding, I would think most teenagers wouldn't think they'd be grouped in that category. An 18-month-old cousin that might throw a tantrum and won't remember it anyway, is different than a teenage cousin who might feel close to someone.
Being excluded from anything is never fun. Being excluded from a family wedding (where there is lots of chatter, excitement, planning, discussion etc for months and months before hand) might be especially hurtful.