I heard this earlier today.

Problem:
What follows the preposition does not refer to a past event or time.

Explanation:
I heard “She’s been competing since she’s been fifteen years old.”` on the Fox News Channel program “Fox & Friends” this morning.

The host was introducing Katie Stam, the latest winner of the Miss America contest.

I suspect that the host’s statement was a gaffe, but the statement is still worth reviewing.

The preposition “since” should be followed by a reference to a fixed, earlier point in time because “since” means between a past event or time and the present.

“She’s been fifteen years old”, which is equivalent to “she has been fifteen years old”, does not directly refer to a past event or time, so it should not follow “since”.

In contrast, “she was fifteen years old” does directly refer to a past event or time. This gives us the solution.

Solution:
“She’s been competing since she was fifteen years old.”