Don't worry about success of your idea if your primary focus is gaining experience. If you get some success as a side effect then that is a nice bonus, but the more important thing is that you gain experience and practice which makes you a success later on when you are working on getting a real job.
Personally when I was your age I was creating websites. I made a book search engine, and a geoip enhanced amazon affiliate ad system for easily adding affiliate ads to blogs and websites. Both achieved some minor success, although they eventually got banned by Amazon (the book search engine because I was scraping Amazon content, and the ad system because it got adopted by a lot of spammy content farmers because it was a highly effective way to monetize Google traffic, so Amazon blocked my API access).
So the ultimate success of my ideas was minimal, but the experience I gained from those two projects got me a great job at a startup that has led me down a very successful career path so far.
Make something you're interested in using yourself. It doesn't matter if the app already exists. Read reviews, see what people do and don't like about it and try to make a better version.
LOL.. at 16, it doesn't matter... you don't have much overhead.. do something you're passionate about.
When I was closer to that age, there wer 16yo running major pirate software networks, art groups, bulletin boards and even developing related software for the fun of it... if someone finds it useful, awesome, if it solves a problem you experience even better.
It doesn't have to be widely used for the experience to be beneficial, and to make a little money off of it.
Personally when I was your age I was creating websites. I made a book search engine, and a geoip enhanced amazon affiliate ad system for easily adding affiliate ads to blogs and websites. Both achieved some minor success, although they eventually got banned by Amazon (the book search engine because I was scraping Amazon content, and the ad system because it got adopted by a lot of spammy content farmers because it was a highly effective way to monetize Google traffic, so Amazon blocked my API access).
So the ultimate success of my ideas was minimal, but the experience I gained from those two projects got me a great job at a startup that has led me down a very successful career path so far.