The fuller answer
There is no medical age by which a person should have had partnered sex. Many adolescents and adults have not had sex, and survey results depend on what acts and populations are counted.
People may wait because of values, opportunity, orientation, disability, religion, anxiety, trauma, no attraction, no suitable partner, or simply not wanting to. None of these automatically means immaturity.
Readiness includes freely choosing, being able to stop, communicating, understanding pregnancy and STI prevention, handling emotional outcomes, and being within the law. Wanting to erase a zero is not enough.