Is an underage relationship a mortal sin for Catholics?

Answer 1

Not in the rule books but it's frowned upon. Don't fret about it though they can't stop you just don't go any further than kissing!

The Catholic Church tries to accommodate the laws of the lands in which its faithful live. As such, the normal legal ages are noted and Catholics are encouraged to observe them. For Catholics, it is an occasion of sin to date without the intent of it leading to possible marriage (that doesn't mean you are going to be married especially if you are just looking - but it means you are not fooling around or just dating for its own sake). Underage relationships these days rarely end up as marriages, or at least cannot be solemnized as marriages until many years have passed, a circumstance which is very hard for purity and chastity to withstand. The relationship will only be mortally sinful depending on: the intent of the parties involved (is for fun, for sex, etc), if it is kept secret from your legal guardians, if it is pursued in spite of just legal laws or the expressed wishes of their legal guardians (as you are underage), if it is scandalous (too much time alone, late nights, i.e. things that give people cause to draw conclusions) or if it becomes irresponsible (making oaths and promises to marry, especially if you do not have the finances or necessary accomplishments to even start a family). If you have any doubts you can consult a priest in the sacrament of confession or ask to meet with him for spiritual direction. He can give you the circumstances in which such a relationship may work as well as those that will not depending on who you are and where you are in the world and in life.