This will not be long, so give me just a moment (most of us don't like "long sermons" anyway). "Mellow" means "ripe, soft, and with good flavor; sweet and juicy." And in the figurative sense, it means, "softened and made wise by age or experience." Remember this definition.
When I was younger, some godly, older men would often say to me, "You are preaching the truth, but the way you say it is too harsh." Thus I learned (hopefully) that you can do a right thing in the wrong way, or putting it in scriptural language, I learned that we should always be careful to "speak the truth in love." I just read a recently printed message on the new birth, and the wise, elderly preacher said this: "And I mean this without intending any offense toward those precious children of God, who believe the contrary system. I have learned, or, at least I am trying to learn, to be more considerate, to be more courteous, than I have in the past toward those precious children of God, who have never come to understand and to appreciate some of the doctrines that are so precious to you and me." He has "mellowed," in other words. Note the kind words, "those precious children of God," and "I am trying to learn to be more considerate, to be more courteous, than I have in the past." Let us all learn here that we must view others as true children of God, even though they differ with us in doctrine or denomination.
What of you, dear friend? I still know some elderly men who need mellowing. We must remember what Elihu said, "Great men are not always wise" (Job 32:9), nor are young men always arrogant and foolish. Grace must temper us, no matter what our age. Especially our tongues and attitudes. There was strife among brethren all through Scripture, but it is not for us to emulate (Genesis 13:8; Luke 22:24). Our only Model is the Humble Servant of Jehovah, our Lord Jesus Christ (Luke 22:27; John 13:15). Men can often make us brash and harsh, but not so our lovely Lord. Listen to these quotes:
"The tongue is in a wet place, and easily slips." "The tongue is not steel, but it cuts."
"The tongue bites sharper than the teeth." "I consider looseness with words no less a defect than looseness of the bowels" (John Calvin).
"But the tongue can no man tame" (James 3:8) -- nor our harsh, unloving attitudes. Only the softening, mellowing grace of God can do that.