Re: Youth and breakfalls.
I've been doing every kind of ukemi possible (it seems like...) for the past fifty-two years and it hasn't done me any harm. I have knee problems, but they are directly related to two things. Really stupid training methods in the early years (didn't we all??) and two accidents. One of them was getting hit three years ago while in a crosswalk by a pick-up truck. Ukemi saved my bacon that time.
My son has been doing similar ukemi practice (without the stupid training methods) for the past 30 years and has no problems other than the results of one training accident. He started training and doing ukemi at a younger age than I did.
Of course, your ukemi training must be based in good principles, etc.
Best of luck in your practice.