Please post your code or attach it. There is no "For" in MATLAB. Plus your shown program uses my_structure and n without ever defining them. So I know that code you posted is not your actual code or you would have different errors.
It could be that there were no ages of exactly 44 for your very first structure so m1 might be either null or undefined.
Don't use i as an iterator since it's the complex constant. Use ii or k or something else.
You don't need to call hold on after every errorbar. Calling it just once after scatter is enough,.
If you have any more questions, then attach your data and code to read it in with the paperclip icon after you read this:
