[17th Miss Regt]

That is absolutely correct. When students begin to be able to think in abstract terms varies by child but it has a profound effect on their ability to learn mathematics. Some cannot do it until they are 15 or 16, others can do it when they are 12 or 13. Seldom does the ability develop before then.

[hinckley buzzard]

You are correct. Jean Piaget determined that the maturation necessary to complete abstract operations begins, on average, around age 16. This has been universally known by competent educators for nearly a century.