wiek dojrzały на английском языке:

Childhood is better than adulthood.

Only children are more likely to grow up geniuses, too, because they also have to enter the world of adulthood sooner.

the next stage in a man's life is adulthood

approaching adulthood

The graduation from elementary school to middle school is regarded by some as the first step towards adulthood.

And speaking of "old age," you can be of mature age and be handsome, well-groomed