In your haste, you fear me. I have broken down the strongest of mortals because they thought they had me under thier control. From the moment you drew your first breath, I was there, and will continue to be a presence in your life until your last days. Some of you take me for granted, some of you cherish me, while others wish they could alter me. I cannot be changed, and will NOT halt my progress for anyone. You often watch me worrying,  but when you don’t I still advance. Children wish me to run faster, while adults wish me to stop to a crawl. There are few who truly understand my value and use me wisely. What am I?