When is an appropriate age to start kissing from a biblical standpoint or is there one?
Most people start at infancy.
Okay, okay, I know that isn't what you meant, but it does point out that it isn't the kiss that is critical to the answer, but the intent of the kiss. The Bible states, "Greet one another with a holy kiss" (II Corinthians 13:12; see also Romans 16:16; I Corinthians 16:20; I Thessalonians 5:26). Many people put the emphasis on the physical activity, the kiss, and thereby miss the intent of the command, which is emphasizing the holiness of the greeting. A kiss is often a greeting given between people of close acquaintances. If you see your Aunt Bertha after several months, you usually expect a hug and a peck on the cheek.
Yet, some greetings can be unholy. When Judas betrayed Jesus, "Immediately he went up to Jesus and said, "Greetings, Rabbi!" and kissed Him" (Matthew 26:49). In this particular case, we would have no problem stating that Judas' greeting was not done with a holy kiss; he greeted Christ with impure motives. Similarly, the soldiers mocking Jesus greeted him on bended knees and saluting him (Matthew 27:29; Mark 15:18), but it wasn't sincere or holy. I point out these negative cases to show that greetings can be good or bad and were done in a variety of ways.
I suspect, though, that you are more interested in the kisses between girlfriend and boyfriend. The Song of Solomon opens with the leading woman stating, "Let him kiss me with the kisses of his mouth - for your love is better than wine" (Song of Solomon 1:2). A kiss is a display of affection between two people.
Yet, we must acknowledge that many people kiss with impure motives. The harlot comes up to a naive young man, "So she caught him and kissed him" (Proverbs 7:13). Here the motivation isn't love. A prostitute doesn't care who she is with. "For by means of a harlot a man is reduced to a crust of bread; and an adulteress will prey upon his precious life" (Proverbs 6:26). The prostitute sees men as a way to put bread on the table; thus, her goal in giving a man a kiss is not to express love but to arouse passion because a man who is sexually aroused doesn't think clearly. "Immediately he went after her, as an ox goes to the slaughter, or as a fool to the correction of the stocks" (Proverbs 7:22). Such kissing between unwed couples of any age would be wrong because sexual passion between unmarried people should not be played with.
Too often young people don't realize or ignore the fire with which they are playing. They try to speed up life and then are shocked with they get burned. The leading woman from the Song of Solomon repeatedly gives this very sound advice, "I charge you, O daughters of Jerusalem, by the gazelles or by the does of the field, do not stir up nor awaken love until it pleases" (Song of Solomon 2:7; 3:5; 8:4). Lasting love develops gently and slowly. When you try to force it, you damage it. Many people want to be in love, so they do the things they "know" people in love do, thinking that this will create love. But the thinking is completely backward. Love must come first, and then the things people in love do naturally follows.
So when is it appropriate for a girl to kiss a boy? When both care for one another, when they are of an age that they are looking for a mate, when they seriously think that this person is the one, when they are mature enough to keep their passion in check until after they are married, and when they wish to express affection and not lust, then it would appropriate to give someone of the opposite sex a kiss.