First let me clarify what I mean by experience and by adult.
Experience in this instance should be viewed as directly applicable and applied knowledge about one or more aspects of the project that are essential to its success. Experience in this definition has little to do with age. There are people straight out of school who may have specialized experience that may be absolutely critical to success, and there may be people who have been around for a long time, but whose experiences, while significant and valuable elsewhere, may not be at all applicable in this project. If you want to succeed, you will need experts with the right kinds of experience.
Being an adult has far more to do with behavior than with chronological age. It has to do with levels of personal responsibility, integrity, and trust. Adults are people who can be trusted to honor their commitments and deliver honest and reliable results in the times they said they would (see Say What You Mean, Mean What You Say, and Do What You Say You’ll Do!). With adults, you don’t spend your time babysitting and resolving stupid conflicts, picking up dropped balls, listening to “can’t do” excuses and other childish behavior. With adults, you get a level of initiative, creativity, and leadership that can emerge and transform the team and project to a new level; breakthrough products can result. With adults, new people in the organization have excellent role models to learn from, and a healthy mentorship relationship can develop that brings benefits to the mentor, apprentice, team, and project (see Learn from Good Role Models; Learn More from Bad!). With adults, you have people you trust to talk to and collaborate with in problem solving. With adults, problems can be foreseen, anticipated, and avoided much earlier and at much less cost. With adults, people rise to the occasion, and delivering on commitments becomes expected and fun. In my career I have known people straight out of school who exhibited outstanding adult behavior, and well-seasoned, even experienced people who still behave like children. Some children (of any age) can be trained, and some cannot! Being an adult does not mean you shouldn’t have fun in your work. Adults can still have fun at work, but not at the expense of others or the project. In fact, having fun should really be a prerequisite, since it is one of the key elements that motivate responsible adults to look forward to going to work every day.