Is it true that calcium a teen takes in from milk and dairy will affect his or her chances of getting thin bones later?

Yes, from any source: Maximum calcium deposition occurs by age 32 or so (peak bone density or bone mass). After that it is generally "topping off the tank". Think of bones as a retirement fund. The more density, the bigger the nest egg when menopause or aging years occur. As such, you need to maintain the "investment" to keep up-so continue the intake of calcium. Other sources incl: tofu, dark greens, quinoa, etc.