Never. LOL There's really no measure really. I know a friend who was married at 20. I was shocked because at the time I was 25 and still not feeling ready. However, she seemed more mature for her age so maybe it was really okay for her. I think being honest with yourself and knowing you are ready is what is most important, not age.
However, I must admit...sometimes...
I wish I would have been married by the time I was 30. It's not too old to have children and old enough to have felt like I did all I could do while single. Then again, I wasn't really sure my ex fiance was the one and I discovered more about myself that I never knew before. As "old" as I was, I didn't know enough about myself yet.
Now, I'm in a live-in relationship w/my bf not yet engaged. I'm on this board mainly because I think it helps heal my memories of my broken engagement and helps me look forward to the possibility of meeting the one even at my age. (I'm almost 38 now.)