If I received money from an estate of my uncle and was sent a K-1 why do I have to pay taxes on the money that was in a 401a if the person was at the age of 80?

Your uncle's age has nothing to do with determining the taxability of this distribution.  The same amount is taxable to you as would have been taxable to your uncle if paid to your uncle before he died.  Only the taxable amount of this distribution paid you you by your uncle's estate should be present in box 5 of the Schedule K-1 (Form 1041) and that entire box 5 amount is taxable to you as ordinary income.