As an F1 student for over 4 years and on OPT, am I exempt from FICA tax?

I am on an F1 visa and will have been in the USA for 5 years on 9th August 2019. I am currently on post-completion OPT. Am I eligible to tell my employer that I am exempt from FICA taxes? What kind of proof can I show? I read somewhere that FICA exemption for F1 lasts only for 5 years.