Absolutely it would be fair to have the same rules for everyone, regardless if they were 6 or 25 or 70 or a professional player, once the players are in the game. It might be _mean_ to treat the six year old like the 25 year old, but it would be fair. It would, however, potentially be unfair to put them in the same game in the first place, especially if there were alternatives.

Meanness and unfairness are two different things.

Especially in a family setting, sometimes niceness can Trump fairness.