http://www.wicca.com
is good
So is
http://www.witchvox.com
which is highly recommended by most wicca on this site.
Good luck. And 21 is a good age, at least you are not of the tweenies that think that Wicca is "cool". It is really a belief for more mature people.
Blessings on your Journey!
You are not too old to learn at all. Look to Llellywen publishers. It would also be a good idea to learn about the cycles of earth from your home point. Also, what type of wiccan you will be. There is more than one kind of wicca and neo pagan. Look to druidism, green witchery and wicca to see what works best for you. It also helps to know someone who understands the religion and its rituals.
No, not too old. Many Wiccans would probably say the older the better. Because, the older you are, the more likely you are to know that Wicca does not equal "Charmed" from TV. You can go to www.witchvox.com and find a local coven that is accepting students in your area. Someone suggested Buckland and another the cnbeyer website. Check out both of those things and read up. That way you can make sure Wicca really does speak to you and not a misconception of what you think Wicca might be. Then, you'll be ready to reach out to a local coven.
No, as a matter of fact covens won't even accept minors largely because it takes maturity to truly begin to understand the religion on a deeper level.
Just remember that first and foremost it is a religion. People who want to love nature or practice magic don't need to become Wiccan to do so; and those things alone don't really amount to Wicca.
A good website is www.cnbeyer.com. There is a good reading list there.
Like any other religion, before you can contemplate joining, you need to know what it's actually about and what we actually believe, and decide if you agree or not.
As many have stated before no actually your a decent age to learn no longer a minor so covens will accept you.
www.witchvox.com is defiantly a good place to start looking for a coven if your interested they will usually help you learn and guide you.
A few beginning things you should look into is which pagan path you want to take there are so many now that, that should be what you should research. From druids, to neo-pagans, to nordics there is a lot to chose so figure out which one calls out to you and go from there.
There are a bunch of good websites out there to chose from just takes a little looking. Remember though the path of the wiccan is the path of the wise research and study is at the center of our religion and if you not willing to always be learning this may not be the path for you.
Also if your decide upon wicca as your path you should start your Book of Shadows and detail your journey and your findings so you have something to go back to.
Totally not too old, and ignore the naysayers, you can study whatever the hell you want. Pick up any book by Raymond Buckland, he's got some good starter stuff. Buckland's Complete Book of Witchcraft is a decent place to start, see if you can get it in a bookstore or online. You can't miss it, it's huge and blue.