The Founders put it in there because they know that with age comes wisdom. They never expected, however, to see octogenarians plus still serving. In those days, people that old had a coveted position.
By the fire place!
And you've probably heard the expression, "There's no fool like and old fool!"
Well, that may be true...I think the downward limit is good, and we have that for a number of things (drivers licenses, for example).
But nobody who started running for office at an advanced age would be elected - unless they happened to be really, truly worthy, and in that case there’s no problem.
The only ones who take their seats in the legislature in their late 70s or 80s (or even 90s) are people who have had way too many terms. Limit EVERYBODY to two terms and there’s no problem.
There are many reasons other than old age for which we should impose term limits. By the 3rd or 4th term, any legislator is going to be so corrupt and bought out by different groups that he or she has ceased to perform any useful function, and in fact that’s one of the reason that they all retire (eventually) as millionaires. In addition, they have stopped younger and better talent from coming up behind them.
2 terms, no more.