His attendance is irregular.

His irregular attendance is disrupting the team's schedule.

He is still in his tsun phase, so he doesn't talk much.

She is in her dere phase now.

He is experiencing the difficulties of puberty.

According to Freud's theory, the phallic stage is a part of a child's sexual development.

Infancy is an important period for human growth and development.

I am a new student of this school.

He missed a once-in-a-lifetime opportunity.

"Once in a lifetime" means to cherish the encounter that happens only once in one's lifetime.