I also don't buy the "Only 14 years old" line. I'm 14, and I've never even THOUGHT about ever doing that. I was brought up around animals, and I love them, and it's hard for me to hurt anything. Hahah, I even try to shoo flies away before resorting to killing them.
We have 2 cats right now (4 cats altogether, 2nd generation.

) and none of them have every tried to destroy our furniture, but here's my opinion:
I am VERY against declawing. I've researched it a lot, and know a lot about it. It is EXTREMELY painful to recover from. Cats walk on their toes, not their feet. It's kind of hard to walk on your toes when you've lost the tip of them. The way a cat is built, the shoulder, elbow, and back joints are all designed to distribute the cat's weight across the toes.
Just like people wearing the wrong shoes, declawing can actually even cause back pain in cats, not just leg pain. There can be nerve damage, hemorrhaging, and painful regrowth of the claw.
Cats also scratch to build muscle, too. So you actually weaken them a bit by declawing.
Do you know how they actually do it? They push on the cat's paw, exposing the claws. Then they take oversized nail clippers and clip the joint off. Wow. Imagine if you went too far with your nail clippers.
Spaying and neutering actually has direct benefits to cats. Well, I'm not sure about neutering, but when you're spayed, you can't get any cervical cancer, et cetera.
There aren't any REAL advantatges to declawing. You can simply trim your cats claws. They have special nail clippers for rounded claws. You can buy SoftPaws, little caps to fit over the claws. You just have to replace them every few weeks. Or you can actually -gasp- TRAIN your cat not to destroy your furniture. You can spray them with a water bottle, or if that's not on hand, then bop them lightly on the head. They hate that, but it's not going to cause them permanent brain damage.
As for cropping ears and docking tails, I don't support those either. But I do think that docking tails is better than cropping ears. Cropping ears causes pain, especially if it's not done right. Almost half the ear is removed, and you have to go to a minimum of 3 months of ear taping.
As for tail docking, I personally think that most dogs would look better with their natural tails anyway.