There is an old saying that age is just a number. It’s true that age is really just a number. Every human is different in that they develop at different speeds. Some are late bloomers while some get an early start in development. Regardless, it is hard to discern an exact age when it comes to pure appearance, though you can guess an age group.
When it comes to immediate threat, one should judge by capability rather than pure age. Being a teenager does not automatically make them a helpless baby. If the teen has a build of a fully grown adult, then he/she is the same physical threat as an adult. Likewise, if the teen is armed and willing to use it, there is no difference than if she/he was a few years older.