A young man can walk at 120 steps/minute
The same man when he becomes old might walk at 80 steps/minute
Why and when does the speed change. The general answer to this is that he slows down as he ages. True.
But, if I can walk at 120 steps/min today, I can surely walk at the same speed tomorrow. And if I can walk at that speed tomorrow then I can surely walk at that speed the day after and so on day after day for years.
So when do we actually slow down?