It is unlikely a youngster will suddenly begin to support himself on his 18th birthday. He should be encouraged, even required to earn money and properly manage it so that he can begin through pre-teen years to buy some of his clothes. An allowance creates dependency. Earned money creates independence. He should be told how he can earn money to get these and more.
Some parents never find the cut-off point.
As a member of the family, he should have definite chores that contribute to the household and should not be paid for doing them. The child is raised when he raises enough money to support himself. One mark of maturity is financial independence. The parents are virtually in the place of God in the life of the small child. However, his relationship must shift from his parents to God.