A big mistake. It does away with the notion that serving is women's work, but doubles down on the notion that it's kid stuff.
Are there no men in your parish? Real men? Altar serving should always be dominated by adult males. Short of being a priest or a husband and father, it's the most manly thing a man can do. This is how you tell the world you mean business. Don't have any real men in your parish? Then get some -- by asking them to step up and own the responsibility that comes with manhood.
We do have a couple of older men who serve, mostly at daily mass but occasionally on Sundays. One problem is the altar server trainer is a woman who prefers girls because they are easier for her to deal with. Again, women’s work.