Does looking at girls invalidate the prayer?

Looking at a woman does not invalidate ablution or prayer, whether she is a non-mahram or not. However, it detracts from the humility (khushu') of prayer. The questioner is advised to lower his gaze and not repeatedly look at what is unlawful, based on the Almighty's saying: "Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do." If the gaze was unintentional, there is no sin in it. But persisting in it is a sin.