Before this becomes a bash-fest like we've seen in some thread, just check his post history. He's already aware he has to wait, and already expressed his desire to do researches before he turns 18.
To answer the questions precisely:
Age verification ensures you really are 18. This allows you to access sections like the used gun classifieds, retailers that sell airsoft pistols and rifles, and other things.
Since "accessories" is vague, and may cover part of an airsoft weapon, the Parts and Accessories section requires that you are Age Verified to access it.
Clothing is accessible to everyone, but I would wait until you are 18 to buy any, since you will grow in the meantime, and your needs will change
As you can see, some people here are helpful but hate to repeat themselves. I would suggest you to read the sticked thread in each category, search using the search function or using Google itself. If you have a specific question that you need answered after than, just come back, show us what you already know, and we'll be glad to fill you about what you don't know.