"At what age do you stop giving your kids lots of Christmas presents?"
Your question has a key clue: "LOTS" of Christmas presents. Giving Christmas gifts is a practice every family has if they can afford it. But over-giving is the best way to spoil any child.
In my opinion when a child stops believing in Santa Claus is a good time to hone down the amount of gifts. The family budget then decides how much to buy.
I’ve seen my own adult daughter give way too much to her 3 children and then has a hard time paying the bills. The kids are spoiled, thinking they’re entitled to the same expensive gifts their friends have (with a two-income family).
Give the child one gift he or she really really wants. Then give them a couple of gifts you think they want or you know they need or will appreciate.
But, also, during the Christmas season, make sure they give something to their siblings and others in the family. Start early in the pre-school years. Don’t let them just be the takers or they may become selfish inconsiderate adults.
It’s a parent’s job to raise kids who become responsible, thoughtful members of society.

