I’ve never heard the title “saint” used for Old Testament characters (although I do believe Adam and Eve were the first into heaven after Jesus opened the gates!). Saints
Sts. Anne and Joachim, two traditional saints who are the grandparents of Jesus, would be older than St. Joseph I’d think, but he may have died first. The Holy Innocents are also saints that died before St. Joseph. Saint Simeon and Saint Anna also died shortly after meeting our Lord when he was a baby. Sts. Caspar, Melchior, and Balthasar might be in the running for oldest, too (the three Magi).
Of course, on consideration, since the Archangels were around a lot longer, they may be the “oldest” saints!