I walk away. What more is there to do? Say mean and hateful things in return?
I believe older parents lose their “filters” as they age... they think and speak much like young children, and just say what’s on their minds. They start to lose the ability to “censure” their words when those words are possibly rude or hurtful. They literally begin to speak without thinking...
You know, all these answer that assume that the oldsters are sane and rational are wrong. A lot of old folks are cranks. You can reason with them forever and they will not get it.
Plenty have some level of dementia, some have other mental problems, and some are simply angry because they do not want to be old.
If your parents fall into one of those groups, often the best you can hope for is to see them on a good day.
I know this sounds stupid, but you don't retaliate. Don't just take it either. Let them know you are hurt/offended. Many hateful things our parents say are usually out of love for us, I know that sounds stupid. I know there are just plain crappy parents, but the majority means well.
explain to them that what they have said is hurtful to you. You appreciate all they have done for you growing up, but now you are an adult and you think you are doing well and are capable of living your own life. You love them dearly, but it just hurts when they say certain things and you respectfully ask them to please not do so any more.