Has anyone over 24 joined the IDF?

i served 7 years in the 10th mountain div, and want to join the IDF, but im 30, id like to get advice from anyone who has gone through this and can tell me what needs to be done to join the Israeli army. im an american citizen, but my grandfather was jewish, and i really want to serve. do i need to become an Israeli citizen, and if so, how do i keep my u.s. citizenship