A little late here but, I feel that one of the factors to consider is how well they shoot the gun. I started both my kids with a .243... Nice lightweight "youth" rifle with a smaller stock, lighter weight gun. They both shoot it well now, but the first year my daughter even found this gun a bit heavy for her (at age 11...) So I feel this does play into the equation.
Let's face the real issue, MONEY. For the deer, the .243 is great, not many argue this. But, for elk....many feel it is too small, I tend to agree. If you were wealthy you would probably buy two calibers?
When my kids transition to elk hunting, I will probably step them up to a .270