I think I’ll hold off on sending those emails. In a way, your examples help the point. Andrew Wiles was noteworthy for proving something so major at such a ‘late’ age, and Terence Tao has been producing brilliant research since his teens. And Wiles had already had a pretty stellar career in his 20s.
I can’t comment on Bhargava. As much as I wish it weren’t true (I’m 53!) I suspect that at least part of it is raw intellectual power which does fade.