Usually, someone Will Hunting's age would be a little old for a coming-of-age story. But in this case, his growth as a person has been stunted by the emotional baggage that he's still carrying around from his childhood.
He's also managed to create a world for himself where he never feels challenged and never has to put his heart on the line for anyone. But these are the exact things that he'll need to grow out of if he's ever going to experience the best things in life. As his therapist Sean Maguire tells him many times, he's going to miss out on a ton of great stuff if he keeps living in the world of books and not experiencing things for himself.
And by the end of the movie, we're happy to see that Will has taken some of the steps he needs to if he's going to move on with his life. Our baby boy's growing up.