A few things

You noted that Miley doesn't look much different at 18 than she did at 16. You are correct. There are girls as young as 12 who can pass for 20 years old. Adolescents are young adults, not children, so this shouldn't be surprising, as physical maturity levels can vary greatly from individual to individual. At this point in the historical record, adolescents simply share a legal status with children until they reach the Magic Age of 18, where they suddenly become adults as recognized by the law.

As for Miley having a lesser maturity level at 16 than 18...it's all an individual thing. Emotional maturity level and experience, like physical maturity, varies greatly from one person to another, and is therefore relative and not universally applicable based on a number. Some people change in great ways emotionally over a two year period, others do not. Some people are very mature and worldly at age 10, others still don't have these qualities to any great extent even at age 35. Many others are somewhere in between full maturity and total immaturity well past the age of 18.

When they say that age is just a number, in actuality that is indeed the case. However, the law doesn't recognize this obvious fact, and our culture likes to say that it does, but it actually doesn't; it largely follows what the law says with its attitudes and stereotypes. If that wasn't the case, then our culture wouldn't put such a heavy emphasis on age 18, which has such a near-divine weight in our culture due to its major legal significance to us.