as early in a man's life
He seems to have had a mental crisis in his 50s. One natural theory is that in the late 1680s or early 1690s he had poisoned himself somewhat in his alchemical experiments, since in that era modern laboratory safety notions had not yet been invented, and many practitioners regularly tasted the results of their experiments
It did not prevent his subsequent election to the Royal Society, and he did run the mint for thirty years afterwards. IIRC there is mathematical work, by other people, from the era in which Newton would have been in his seventies, which Newton is believed to have helped produce
I think his problem with hecklers occurred early in his life: he seems to have been so gravely discouraged by opposition to the theory of optics he began to devise, that he abandoned the theory. And after Halley and others finally persuaded him to publish his astronomical mechanics, he indicated he had deliberately written it in a somewhat-difficult-to-understand way, so that the hecklers would have trouble understanding it
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