The first recorded example, according to the Internet seems to be 1637.

It is very unlikely that this was the first time it was used, and how can we be sure it meant what we take it to mean now.

Many reasons are speculated as to why this came about, most are mere speculation using modern iconic references and symbolism.

There are two reasons as to why this association came about that stand head and shoulders above the others.

The first is that when documents were signed by illiterates in the middle ages an X was considered a legal mark, to show their sincerity the signee would kiss the X they had signed. If this is considered the first use of the symbol to represent a kiss the logic is flawed.

The second is later when lovers would write to each other, as a sign of affection they would kiss the letter by their signature, then mark the spot with an 'X'. By this method the recipient coud receive the kiss by kissing the same spot - now that makes so much more sense.

When did it first happen?

At a guess in the early Victorian era (coronation 1937) when more of the population were beginning to be able to read and write.