Though a vegetarian or vegan diet can provide all of your nutrients there are a couple of important things to watch for.
I was talking to a friend who mentioned that many of the vegetarians she knew switched back to eating meat after several years because they felt tired. A bit later I realized that sounded like a B12 deficiency, it can take several years to show up and the first symptoms are feeling tired. (Also as we age, it becomes harder to absorb the B12 present in meat, so meat eaters might want to consider adding a B12 supplement in their late 40s.)
The other thing is after reading about all the health advantages of well planned vegetarian diets, it's good that I'm being friends with people noticeably younger than me, since all those meat eaters will be far more likely to be developing heart disease, diabetes, and cancer than me.
And also the comments about vegetarianism possibly also helping with rheumatoid arthritis also rings true. Many of the meat eaters that I know complain about joint pain and how they're rapidly decaying. The only thing that's wearing out for me is my vision is slowly worsening.
I want to make their cashew french toast tomorrow.