26 year old dudes are adults, full stop. And in my experience, they don't think nor act like teenagers whether they settled down or not, whether they married or not, whether they kept old hobbies or not.
People "reach adulthood" (to use your words) when they take on real responsibility. Some people never do this. Some do it at an early age.
Some things that often cause people to grow up fast include having children or serving in the military.
(Those are opinions. Source: Me.)