I'm going to tell my parents they're selfish for not saving for retirement
Do you think saying that would change their minds?
They're not going to change their minds unless I get on my knees and beg them
Insulting people or begging will not change their minds. It's their retirement and they can do it how they want. It has nothing to do with you.
I don't want to take care of them in their old age.
Telling them that the are selfish will just make them dig in their heels. Insults usually aren't argument winners.