You have to be 18 years old or older to legally buy sharpies.
Any legit store will not sell you a sharpie marker if you're under the age of 18 years old.
The reason you have to be at least 18 years old to buy sharpies is the sharpie markers can get you high and can kill you.
Some under 18 year olds have been using sharpies to get high off the chemicals in the sharpie marker which can potentially kill you.
The people will sniff in the chemical and get high and can pass out.
Another reason you have to be 18 or older to buy sharpies is teens have been using sharpies to deface things such as writing on buildings and inside of bathrooms etc.
In some places you could be ticketed if you're caught with a sharpie in public under the age of 18.