I can't speak for that frame in particular, and in any case it's impossible to reason from the general to the specific.
BUT
Steel frames very rarely fail when there's no upset (dent, buckle or bend) from a high impact. It does happen from time to time, especially in highly stressed top end frames, but even that is rare. I'd be perfectly comfortable riding that bike if it's never been crashed (with visible evidence), and I weigh quite a bit more than you do.
So, I could almost say steel frames last forever, but it's not quite true. So while they have near in finite lives if nothing major happens, the only sure way to never have a frame crack is not to ride a bike at all.
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