Yes, one should fast is he/she is able, but it is considered a “minor” fast day and therefore heterim are available for those who need them. My mother is not elderly and does not have an illness, so she has to fast on Tisha B’Av and Yom Kippur, but after nearly fainting on Sheva Asar B’Tammuz a few years ago she does not fast on any of the four minor fast days anymore. I also have difficulty fasting, but not to the extent that I can justify not fasting. I just make sure to get plenty of rest and not to go out in the heat.
