You can use whichever you want. Both are correct. But if you want to choose the right one, it is 歳. 才(SAI) has been introduced to substitute 歳(SAI) officially (it is called 代用漢字）, because 歳 has so a complicated form that it takes much time to write it. While 才 was chosen because it happened to have the same sound as “歳“ and require only three strokes to write it, the meaning of 才 has nothing to do with the conception of age.