suitable age; appropriate age
Often used in fixed compounds like 適齢期 (marriageable age) or 労働適齢 (working age). As a standalone noun, it refers to the age considered right for a particular activity or stage of life.
She is reaching the marriageable age.
There is a suitable age for this job.
She feels she has reached the marriageable age.
He has already passed the appropriate age for finding a job.