From what I can see Martin Van Buren was the oldest living US President at the time of the Civil war, do we have any records on his thoughts on the time given he was born During the Revolutionary war and to die during the Civil war.
check out /u/Chaingunfighter's response which deals with how all former presidents responded to the civil war.
https://www.reddit.com/r/AskHistorians/comments/52476q/how_did_former_us_presidents_who_were_alive/
Martin Van Buren also openly supported the union. In the years leading up to the war, Van Buren fought staunchly to prevent southern secession, if ultimately unsuccessful. He responded probably most harshly of all former presidents to the Confederate bombardment of Fort Sumter: "The attack upon our flag and the capture of Fort Sumter by the Secessionist could be regarded in no other light than as the commencement of a treasonable attempt to overthrow the Federal Government by military force." Van Buren, despite being strongly against slavery, was one of few Democrats not to switch to the Republican party in the 1850's even as the Democratic party began to support the institution. Though initially supporting Stephen Douglass, the 1861 Democratic candidate, the war's onset eventually moved him to support Lincoln. When confronted by Pierce about meeting to use their presidential influence to negotiate an end to the war, Van Buren refused, believing that Pierce's peace terms included allowing the Confederacy's existence to continue. Van Buren died relatively early in 1862 as the conflict ramped up.