How can I strive against the absurd thoughts that come to me when I begin to worship, and is an adolescent held accountable for his actions?

These whispers and evil thoughts are from the plots of Satan, and you must strive against yourself to repel them. It is from Allah's mercy that He has forgiven this Ummah for what they thought to themselves, as long as they did not act upon it or speak of it. Therefore, you are not held accountable for them unless they become intentions, then transform into firm resolves, and then become actions. In that case, a servant is held accountable for them after reaching puberty. Guarding thoughts and preserving them protects the heart from the seeds of evil and prevents Satan from gaining control, as indulging in them can lead to undesirable consequences.