The old man is too old to walk fast.(too...to ...结构,表示“太...以至于不能.”)
The old man is so old that he can't walk fast.（so...that...结构,表示“如此...以至于...”）
注意区别：1句句型已带有否定含义,所以后不加not,2句不带否定含义,所以还要加上not.
希望好好学习英语啊,加油!