Best Answer - Chosen by the Asker
Your sentence does not convey what you meant to say.
I would suggest the following,
Cantonese (colloquial): 就算你幾多歲都好，你都一樣咁靚。
Chinese (written): 不論年紀，你都依然美麗。
Regarding the word "Beautiful", I think most of the time you can use 靚 because it is used in describing people, things and scenery. Other descriptions, such as 索，正，散鏡 mainly apply to describing people (particularly hot ladies). They are more like slangs so I don't really recommend you to use those words.
I hope your mother will be delighted when she receives the card. :)
