Back in the day, "yaşımdayım" was correct. Nowadays, we use "yaşında" for all persons.
I think it may be along the lines of "You have an age of..." Yasin = age. I have the same question as you, so if someone could clarify (in English, please), that would be terrific.
So, it literally translates into "You are in your fifty fourth year". right?
Note to course creators: I mistakenly typed 34 in my sentence instead of 54 and it was accepted as correct with a typo. I expect numbers written numerically should be exact?