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Not clear to me if they're 45 when it's founded or when it becomes successful. Sounds like it's the latter.

Among the very fastest-growing new tech companies, the average founder was 45 at the time of founding.

It says "at the time of founding"

I will chalk it up to my allergies.

I think the former: "Among the very fastest-growing new tech companies, the average founder was 45 at the time of founding."