Legal Definition of Maturity: (a) The end of the time period agreed upon for a loan or obligation, after which time the loan or obligation is due and must be paid. (b) The attainment of adulthood. A child is said to reach maturity when he or she attains 18 or 21 years of age | Discover the Legal Definition of Maturity | Learn About Loan Terms and Adult Responsibilities. Explore examples and the importance of maturity in financial planning and legal rights. Plan your finances and navigate obligations confidently with a clear understanding of maturity. Click to know more