Like Cai said, it really is a personal thing.
I must admit I get a little upset when people jump in and discipline my son before I've even had the chance to walk over there, but if I wasn't paying attention and didn't see the incident take place I would be ok with someone else telling him it wasn't nice to do that etc. I do have a problem with people being more stern than I usually am, my DS is 18 months as well and I feel that he is still learning the whole concept of sharing, being polite etc so there's no need to be stern, just a simple "no we don't do that" or "we need to share that" would be fine for a child of his age.
Hope that helps a little