The people who wrote the Constitution wanted to follow, in a way, the British system. In Parliament, there was a House of Commons, elected by the people, and a House of Lords mostly hereditary. The House of Lords was supposed to be wiser than the House of Commons. Similarly, the Senate was supposed to be wiser than the House. George Washington once said the Senate was like a saucer. People would pour tea into a saucer to let it cool. In just that way, legislation would be "poured into" the Senate to let it cool.
In the Senate.gov link below, James Madison explains this in a more straightforward way. He says that the Senate is supposed to do its work
with more coolness, with more system and with more wisdom, than the popular branch.
The Senate, then, is supposed to be wiser and calmer. This is why Senators are supposed to be older than members of the House.