the already-old-before-my-time age of seventeen

[Vladv1]

"Hoping for a break, any kind of a break. And where, somehow, in 1960, at the already-old-before-my-time age of seventeen, I was allowed to record a Tin Pan Alley ballad-by-numbers called “An Angel Is Missing.” That was as Ronnie Dio and the Red Caps. It was not a hit. But it didn’t stop me then, and it doesn’t stop me now."
Rainbow in the Dark - Ronnie James Dio
What does "the already-old-before-my-time age of seventeen" mean in other words?

 

[emsr2d2]

It means that when he was seventeen, he was considered (or considered himself) to be "old before his time". It's used to express that he didn't fit the stereotypical idea of a seventeen-year-old. Either his outlook or his experience made him seem older.