Technical definition of adulthood (per AHA anyway) is onset of secondary sexual characteristics.
An adult dose of adenosine in a 45-50 kg patient is not going to be detrimental. I only mention adenosine because we happen to be talking about it at this point. Please don't take this as me making light of using best practice when making dosing decisions for our patients. In this case however, given my experience, the effect is negligible.
16 year old patients are routinely managed at adult hospitals in non pediatric wards, and in some cases are transferred away from pediatric hospitals depending on what the nature of illness or injury is. It all depends on your system I suppose.
At any rate, while some protocols may use an age definition for adult v. pedi, and some use onset of secondary sexual characteristics, the practicality of either method is lost in the field if providers aren't allowed to use common sense.