The Senate shouldn't have term limits. Why? Because the Senate is LITERALLY meant for old people.
The word "senate" comes from the Latin word "senatus" which LITERALLY means the "assembly of elders." The Senate in Ancient Rome was all old people for example.
The word "SENior" and yes - even the word "SENile" - originates from the Latin word "senex," which is how they get SENator as well.
So the Senate IS supposed to be full of the elder folks of the government.
Also I don't think anyone wants a legislative house full of 6-year long lame ducks.
