system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do OTC medicines go by age instead of pounds?

Got my 6'3" son cold medicine and it has dosages by age instead of weight. This makes no sense. It's been bothering me for years. All OTC meds do this, but prescriptions go according to weight.

Any ideas why a petite woman would get the same dosage as Terry Crews?
gpt
Because biology is complicated, and going by weight isn't that much more accurate.  

Let's say Terry Crews's body is twice the weight of your typical Olsen twin's.  Is his brain?  His liver?  His thyroid gland?  Is the drug used throughout his body, or is it simply triggering a response, and beyond a certain amount doesn't matter?  What if the twin started porking out until she weighed the same as Terry?  Her organs didn't suddenly get bigger.  What kind of math would you have to use to take all these factors into account?

Instead, drug manufacturers play it safe and give the appropriate dosage for a small adult.  They will let doctors and patients assume any risk of increasing that dose.