I will answer your questions in reverse order because I think that will make the most sense.
does juggling only improve the control of the instep?
It depends on which part of the foot you are using to juggle with. You would not expect to learn how to do linear algebra by reading a 5th grade textbook would you?
If you wanted to improve the control of the inside of your foot then you would juggle with the inside of you foot. If you wanted yo improve the control of your head then you would proceed to juggle with your head.
Since we're talking about the benifits of juggling I may as well get into the details:
If you are able to juggle 413 times then you have moved beyond the benefit, however small (I'm not a fan of counting juggling), of counting juggling. So if you wish do a type of juggling that is more beneficial then I reccomend rythm juggling. The concept is exactly as the name implies: juggle in rythms, or patterns, such as instep, instep, head; do this (or any pattern that you've made up) for maybe five minutes and it's as simple as that. If you do these exercises for as long as you did counting juggling each day then you should see much better improvement than you would have see with counting juggling.
how much should u do at age 15?.
By the age of 15 you should be able to do at least 300, but that's only if you actually had that type of goal. The truth is that you don't have to be able to juggle worth squat to be a good player, so you might only be able to do 7 and it wouldn't make that much of a difference. Juggling is a complementary skill and is not as essential as something like shooting or dribbling.
In other words, if you set aside a substantial amount of time to work on juggling and you still can't do 300 by the age of 15, that is not good.
If you didn't work on juggling but instead worked on another skill and you can't do 300 by the age of 15, then that's probably alright.
is that good for my age?
Yes. That is probably above the average player... As long as we're bragging though I could do 311 when I was 11.
Anyway, I hope this helped you.