Well if I'm not wrong, usually when a word has multiple kanjis one of them is selected as the "main" or more commonly used one.
But is it true that 才 and 歳 are both the "main" kanji for さい?
It seems that 歳 is the "official"character for the age, even though both it and 才 are reglementary (常用漢字). However, it is too difficult for the pupils (小学生) who are supposed to learn it since it's a very common word. Therefore, the different (but not simplified) character 才 is taught instead so that they can learn a necessary character until they see the "hard" one a few years later.
So, the "official" one would be 歳. Same goes for 令 instead of 齢
I agree with Axioplase, but
If I say, "Because I'm getting old," I will use