You worded it fine. You could also have said this: "By the time any of my students are my age, they will have probably had a child." Both sound correct. Edit: Although there is a mild difference in meaning. Yours is that the students had the action of having a child (birth). Mine is that the students have children (are parents). Regardless, there's a lightly unnatural tone to the sentence. It has to do with the word "any." I would say "...any one of my students..." sounds much more natural.

I think it´s ok