This would only hold if the demographics stay the same. In 10 years time, half a generation of children will grow into their teens . If for some reason there a

This would only hold if the demographics stay the same. In 10 years time, half a generation of children will grow into their teens . If for some reason there are more/fewer biologically female teenagers, you might expect more/fewer teen pregnancies. Moreover, the teen pregnancy rate is impacted by many unrelated factors (sex education, social mores, etc). You could try to model the change these factors might have, but starting from actual demographics gives you an additional piece of information.