Is becoming a vegetarian/pescatarian a good idea at my age?

It's fine at any age, as long as you are able to get the proper nutrients from the right foods (as in knowing what these foods are, and are able to buy them or have your parents buy them for you). There are many people who were raised vegan/vegetarian without any negative consequences what-so-ever. It would be a good idea to see your doctor a couple times a years for the first few years to get blood work done so you know if you are missing anything important. Do a lot of research before you start so you have an idea of what a healthy vegetarian diet looks like. You're going to want to eat a lot of whole foods, and not just over-processed meat replacements. Make sure you eat a variety of vegetables, and lots of them.

Some plant-based sources of protein include:
Beans and other legumes - kidney, black, chickpeas, lentils, peanuts, peas...
Whole-grains - bagels, cereal, oats, bread, quinoa, barley, brown rice...
Soy-based foods - tofu, tempeh, milk
Seitan - wheat based, 75 g protein per 100g (150% of your daily intake)
Vegetables - Brussels sprouts, spinach, cauliflower, mushrooms, broccoli...
Nuts - almonds, cashews, pistachios, nut butters...
Seeds - pumpkin, chia, hemp, sunflower...

There are plenty of websites out there with a ton of useful information. Some will even send you a free vegetarian/vegan starter kit in the mail. https://www.peta.org/living/food/free-vegan-starter-kit/

Good luck. :)

Becoming a vegetarian at such a young age is not a good idea. You should probably wait until you've stopped growing and until you're old enough to make sure that you can buy all of the foods that your body will need.

It's ONLY fine if you are a NUTRITION guru, so you know how to eat a balanced vegetarian or pescetarian diet, capiche?

If your parents agree and will support your decision, then it's fine. It won't hurt you if you eat a well balanced diet. You could learn to cook for yourself, and help with the shopping.

It is healthy at any age if done properly and you do plenty of research. My children have been vegetarian from birth and two of them have now chosen to be vegan, they have had a very healthy childhood, hardly a day off school. Log on to the Vegetarian Society website and claim a free starter pack.

Becoming vegetarian is never a good idea. At least, that is my opinion.