No spiritual believer should ever be critical of a young believer who addresses God as "You". We have often heard new believers address God in this way, but their warmth and devotion touched our hearts very deeply. However, for older believers, we should expect that they would use the reverential "Thou" when addressing God for the following reasons.
The personal pronoun of the second person singular in the NT is thou, "son". "Humas" is normally used for an object, which is undefined as to its number. God is never addressed as "You" (humas), nor is this word used generally as a form of address.
Even the King James Version translators were careful to note this distinction throughout the Bible. A plain example is in Luke 22:31-32, "And the Lord said, Simon, Simon, behold, Satan hath desired to have you (plural - humas), that he may sift you as wheat: But I have prayed for thee (singular - son), that thy faith fail not: and when thou art converted, strengthen thy brethren." So Scripture does distinguish the difference between the plural "you" and singular "thee". God is never an ambiguous plural, He is The Singular. The clear statements of Scripture are: "Hear, O Israel: The Lord our God is One Lord" (Deut 6:4). The Lord Jesus called these words the first of all the commandments, "The first of all the commandments is, Hear, O Israel; The Lord our God is One Lord" (Mark 12:29). He said, "I and My Father are One" (John 10:30). Paul wrote, "Now a mediator is not a mediator of one, but God is One" (Gal 3:20). Why then should we use an ambiguous pronoun that can mean one or many, and is never used in Scripture as a form of address in addressing Him? He is uniquely One.
No argument from the original language of Scripture can be given to prove that God was addressed with a different singular pronoun from an address to another person like us. However, our English language along with all of the Romance languages allows us to use a unique form of address to God. If some argue that it is not reverence but tradition, we in turn argue that it allows us not to address God as we would an equal, but in a unique way that permits us to express our awe of His holy presence. It is a great privilege to show reverence with language and it should never be allowed to deteriorate into a mere formality.
It can be proved that the Lord Jesus showed reverence by means of language. When He taught His own how to make requests of the Father, He used a word that means a lowly petition in recognition of our unworthiness and that God our Father is the Almighty (aiteo, John 14:13-14; 15:16; 16:23). However, when He Himself made request of the Father, He used a word that means to ask as an equal. "And I will pray (erotao) the Father, and he shall give you another Comforter, that He may abide with you for ever" (John 14:16). We use "you" in addressing an equal, but we have the great privilege of using "Thou" when we address the Lord of All.
When US President Bush was being inaugurated, two clergymen prayed and addressed God as "You". To my ears, it was very irreverent, and in all their form of address there was no sense that they were addressing the Almighty, and they never mentioned the Lord Jesus. Then Dr. Billy Graham prayed. He said, "Our Father in Heaven, the God and Father of our Lord Jesus Christ, we thank Thee..." The difference was amazing and I thanked God for his "old-fashioned" reverence.
There is another principle that applies in how we address God. We should use great care in not offending others. "Give none offence to the church of God" (1 Cor 10:32).