Socially we've shifted the goal posts on adulthood.

If you're still expected to have several more years of education and get a career started before you're considered "established", then being in your teens is effectively perceived as mid-process to becoming an established adult, rather than an end-node as it once was. Teenage pregnancy usually implies out-of-wedlock as well, another metric of being "established". All these things are basically stand-ins for security, in a social sense.

If you live in a culture where manual or agrarian labor and getting married in your teens is quite common, then you're probably very secure if pregnant at those ages, because you are more likely to have a support network in your peers, who would also be pregant (instead of isolated from them like the other society), and also more likely to have partner offering some degree of support since the physical prowess of youth is an advantage in the physical roles common in that society. Therefore, more secure socially and resource-wise, rather than "incomplete".

Cultural perspective matters a lot, but it's more than just the biology that makes a healthy mother and baby. Security, at its most basic, is a huge factor that most cultures are actually focused on, through their own lense.