If i'm 18 years old, can i get a blod clot in my leg?

Yes: It can occur at any age but is very uncommon in an 18 year old.

While unusual....: It does happen. There are many risk factors that predispose a person to clot, some constitutional and some acquired. We all have a thrombotic threshold, and the accumulation of constitutional and acquired risk factors bring us ever closer to it. As such any person, depending on their aggregate pro thrombotic risk, can develop a clot under the right circumstances.