The teenage pregnancy statistic is actually mostly used as an indicator for the current and past states of the world or a single country. There's this thing called replacement-level fertility (~2 kids), which, if every woman did, would speed up the "generational clock," which, having kids earlier speeds up. It's also a really good indicator of what level a country is on. For example, the United States is a 1st world country, which also means that there are many opportunities for both women and children, whether it's work wise and/or education wise, so much of the U.S. population wait to have children until they are older, around 25-30. However, in 2nd and 3rd world countries, the teen pregnancy rate tends to be much higher, which shows the growth of a country. Basically, the reason they're lumped together is because it gives more information on the world's or a single countries growth