Suppose you have a 31 year old son who's living with you. He has a college degree, a stable job that pays 100K/year, and plenty of cash in the bank. He pays his fair share of the rent and utilities, helps around with the household chores, and rarely if ever asks for help. He doesn't have any harmful addictions and is considerate (i.e., he doesn't blast the music at 2 AM and leave his room in a mess).
He has a wide variety of hobbies but doesn't have any close friends since almost everyone he knows is busy with their wives and kids. Although he's been to many different states and countries, he wants to continue living with his parents because the environment is familiar to him, and he likes their accompaniment.
In that case, would there be anything wrong with letting him stay with you indefinitely? Or would you encourage him to move out?