I think I have found it for you here, though it is from a French rather than an Italian Renaissance writer, Michel de Montaigne. He is referring to a story about a Greek philosopher, Thales of Miletus, who lived in the 7th century BCE:

"Thales gave the truest limits, who, young and being importuned by his mother to marry, answered, 'That it was too soon,' and, being grown into years and urged again, 'That it was too late.' A man must deny opportunity to every inopportune action."