What do you think my handicap will be?
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There's no way to answer this in any sort of useful manner, so let me guess a number purely out of speculation:
I think you're capable of playing to an 8 handicap so I think you'll still be right there as a senior. If you practice diligently and get good guidance, you could go a bit lower. Getting below about a 4 is hard though so you'll need to practice really diligently and get really good guidance to do that.
You can only answer this yourself. What do you want us to tell you when we don't know your game, background, or level of commitment?
You need to work on your long game about 65% of the time. The short game is usually the easiest way to lower your score but you can't lose as many strokes as working on your long game over the long term.
Not saying that will happen to OP, or that he doesn't need to practice. But simply being 15 can really hold you back, and when you're 18 you don't have the same limitations.
If you're still growing as most teens are, you may grow into additional lenght which means lower scores usually.
I play often with one of the top ranked 14 year olds in Florida. The kid gets better every day. He drives the ball in the 300 yard range already. He plays with the top golfers in his area. 1 year ago he was shooting 80, now he's usually close to par.
So young players can improve dramatically.
Stay focused and work with your coaches and good instructors. Don't focus on the handicap number. Focus on improving.