저는 열네 살입니다 (jeoneun yeolne sal-imnida)
* 저는 (jeoneun): I am
* 열네 (yeolne): fourteen
* 살 (sal): year(s) old
* 입니다 (imnida): polite form of "to be"
You could also use the slightly less formal:
열네 살이에요 (yeolne sal-ieyo)
This uses the "-ieyo" ending which is still polite but a bit more casual. Choose the first option if speaking to elders or someone you don't know well.