Good luck with your enterprise, it is good to know that there are still young fellas out there who are willing to get stuck in and have a go.
This job can be a very good number for a fit young bloke, and if you get to it early you could go all the way and develope a very succesful business.
You seem to be a little daunted as to whether you will be taken seriously as a 15 yr old, so the thing to do is to get your message across quickly and effectivley. I would sugest having a flyer made up with a picture of yourself actually doing the job, and a brief note explaining that you intend to follow the family business. When you are out door knocking, be prepared to do the job there and then.
People like enterprise, especially in todays climate, I think you could do quite well.
I see that you are planning to work trad, however there is plenty of cleaning that can be done with a wfp and garden tap. My absolute best advice is that you consider getting a wfp and use it to clean/ sofits/ gutters/ walls/ sills and frames before finishing windows trad. There is good money to be made washing these things, and the water does not need to be pure, so a garden tap and custy hose will do. Also a pole would bunjee to the ladder and be easy to carry about.
Many years ago I started cleaning trad and didn't even have a ladder. I started on neighbours downstairs windows and promised to come back for the tops when I got one. I was given one soon enough, and set about all the tops in the area until I could afford a car. I scrounged a cheap car from a local dealer and in the three months I had until the mot ran out, I worked my conkers off and was up and running.
Good luck, I hope it goes well for you.