Ruining your life can mean many different things to a young girl. They can drop out of school, have a baby too young, find the wrong man that only beats on them, or resent the fact they didn't do what they wanted to do before they got married. All this to a young girls makes them feel they have failed and ruined their life.
There are many peer pressures in school when you are around 16. One of them is finding or having a boyfriend that goes to the same school as you do. Other peer pressures could be to smoke drugs, cigarettes or even drink. The pressures from friends at school could cause you to try or do something that you would never have done before. In some cases, these young girls feel guilty about what they've done and felt they have let down their families. In cases like this, the young girls will feel she has ruined everything and she is no longer pure or fit for anyone.
In other instances when a young girl is forced to marry a man that she doesn't know, have sex, make a family and take care of him, they feel like their life is ruined and there is nowhere to turn and they are stuck in an unhappy relationship or marriage.
This is sort of a broad question and can mean so much. But if you are referring to a young girl having sex for the first time, this will all depend on them and how they feel. There are some girls that are pressured into having sex at a very young age. In some cases, the girl is fine with this and others it makes them feel bad or guilty. However, if they happen to end up with a baby and the family doesn't accept this child, then the young girl is left on her own to try and work and take care of the child. In this instance, they feel like their life is doomed and they have no hope at all.
When a girl marries young and she has chosen badly the man for her life, the marriage ends in divorce, and the young girls feel lost, left alone and that she has ruined everything.
So you see there is no real age this can happen, it can be as young as 12 and as old as 22. In all instances, the person feels their life is ruined and there is no hope for them.