First option is "under 15". From this you can infer pretty straight forwardly that the next option "15 - 20" means ">= 15 and < 20". (why would he skip the age 15?) ( But you have to read the last option pretty liberally :-) )

Now that I think about this, I realize that there is always one guy in every class who sits in the first row, lifting his hand to ask a question like this :-)