Look for your state board of pharmacy laws and rules - it is quite likely that the pharmacist is referring to company policy and not state law. I know Texas requires you to be 17 to be a "technician trainee" and 18 with a HS diploma/GED to be a registered and certified technician. However, there are no laws regarding shadowing - but many companies/independent pharmacies have rules.
Reach out to what is called the PIC (pharmacist-in-charge) of pharmacies (retail and independent). Regardless of the situation, they will be the ones that ultimately have to make the decision to let you shadow, and they are in a role that will mean they are more likely to understand your situation. Texas state board of pharmacy has a website you can look up the PIC for the pharmacy - so you can at least know their name when you go in. Look for a similar site for your state.