My colleague tells me that I usually get sick because I did not complete a year of breastfeeding from my mom. Is it true that breastfeeding affects the immune system forever?

It may be true that breastfeeding affects the health of a person for a lifetime but of course, it can be influenced by other factors. No matter how long a person has been breastfed, if his/her lifestyle is unhealthy, then I guess that can affect a person a lot.