Should you start a company as a 14/15 year old when you have got a good idea an you are able to make it real (Design, development, etc.) ?

Yes! I ran a silk-screen printing business when I was 15. It was a great learning experience.

Do you ever watch Shark Tank? This episode has all kid/teen entrepreneurs:
http://www.huffingtonpost.com/2014/03/12/shark-tank-kids-episode_n_4950983.html

Here's a Ted talk check out:
https://www.ted.com/talks/maya_penn_meet_a_young_entrepreneur_cartoonist_designer_activist

Remember, most adult entrepreneurs have no idea what they are doing! We try things, see what sticks, and learn from our mistakes.

Of course!!
For the legal aspects you might need an adult like your mom or dad. Someone you trust, try forming an llc as a partnership so that both have control and access to decisions.

Now, in regards to your schedule and ability to focus and commit; lets face it you are a teenager and thus have to deal with other things that will want to grab your attention so my only suggestion is to be careful in biting too much and then having a business you either cant handle because of school, parents, etc... Or simply not interested anymore to commit as much as it demands.

If you have trouble ideating or fine tuning and if your working on a novel idea:
Work on an elevator pitch:
Crafting an elevator pitch is a tried-and-true tactic for people who have trouble getting to the core of an idea. But gor your business people your age, this helps look more professional, and allowd you to sway people your way for investment or etc.
A pitch: developing a 30-second description of your strategy can help you think about how you’re going make things happen. Use these three questions to build your pitch:
1. What’s the conflict?
2. What’s the impact of the conflict?
3. What’s your resolution?
Also, thr problem usually gives you the solution.

Email me if you want mote free guidance! Id love to be a mentor in what i can! Iwill@unthink.me

Yes. I started my first company when I was 13. It was because of my early development efforts and consistent outreach that got me a full time job.