I've found the statement that you can't shop at Target unless you're 18 is exaggerated and misleading. During my research, I have found that while there have been instances at different Targets, there hasn't been a nationwide Target policy put in place. There are some blog-style sources, such as The Break Room, where people have said their local Target had to put a policy in place to stop minors from shopping there. After reading through Target's terms and conditions, I didn't find anything that explicitly states that you must be 18 to shop at Target. As I began to delve deeper into my research because I was curious as to why some Targets would enforce a rule like this, and interestingly enough, one article pointed out that it was a product of the pandemic. The Street stated that the pandemic had shifted the way people look at shopping, specifically grocery shopping, so places like Target had to shrink inventory, and to prevent selling out too soon, some Targets had to enforce an age limit. While that was the case for some, others were simply running into consistent trouble with the minors in their area and had to find a way to stop it. Basically, some Targets have enforced this law, but it isn't a nationwide rule for Target.
Sources:
https://www.thebreakroom.org/threads/is-your-target-store-cracking-down-on-minors.31963/
https://www.thestreet.com/retail/target-store-cracks-down-with-over-18-policy
https://www.target.com/c/terms-conditions/-/N-4sr7l