Based on Christ's statement, when people (male or female) actively think about having sex with another person who they are not married to it is always sin, even when they do not actually commit the act.
This principle is valid regardless of the age of those involved, as both young and old can lust (although the young normally do it more than those who are older).
There are many gradations in-between these two extremes, and it is possible that what is sinful for one person will not be for another, depending on their thought processes.
Furthermore, sustained kissing is considered foreplay as preparation for physical intercourse.
It can easily be a sexually charged "warm-up" that prepares for consummation even when the couple ultimately avoids going that far.
Sometimes it is not so easy to put this act in a watertight compartment that is completely separate from sex.
One reason for single and dating Christian people to avoid strong romantic kissing is the need to protect their own and other person's purity for their (likely) marriage to someone else.
But living a just and holy life requires one to be capable of an objective and impartial evaluation of things: to love things, that is to say, in the right order, so that you do not love what is not to be loved, or fail to love what is to be loved, or have a greater love for what should be loved less, or an equal love for things that should be loved less or more, or a lesser or greater love for things that should be loved equally.
(On Christian Doctrine, I.27-28) of sex (within or without marriage) sinful in any way, we thwart the beauty of it at all; we mar an act that God has called good.