According to the Church Fathers or other Catholic theologians, how old was St. Joseph when he married the Blessed Virgin Mary?
According to ch. 1 of The Divine Favors Granted to St. Joseph (1973) by Père Binet, S.J., the general opinion among theologians is that St. Joseph was, when he married, "neither an old man nor a youth, but in the prime of life, between thirty and forty":
A secret inspiration from heaven caused both Mary and Joseph to contract this alliance, while adoring in their hearts the impenetrable counsel of the great God. Mary was in her fifteenth year; the age of Joseph is not known so exactly, tradition being silent on the subject. The opinion that he was about eighty years old is without reasonable grounds, and is not held by theologians, the most esteemed of whom think that he was neither an old man nor a youth, but in the prime of life, between thirty and forty. There are many reasons in support of this opinion, which is now generally held.
The origin of the "old Joseph" and "young Mary" is in the Infancy Gospel of James, per Origen. Scripture implies they would have been about a similar age.
Some 200 years later, Jerome was aghast at the idea that Joseph wasn't also understood to be an ever-virgin. He then invented the cousin theory to explain the nature of the brothers and sisters of Jesus. Cousins, rather than children of old Joseph from a previous marriage when he was younger.
So, to answer the OP question, the answer about ages will depend upon your source. Scripture (implies about same age), Infancy Gospel (old Joseph, young Mary), Jerome (unclear, though probably about same age, but no consummated marriage). To be sure, there are many others (Origen, Valentinus, Tertullian, Clement of Alexandria, Cyril of Jerusalem, Hegesippius, and others) who opined about this issue.
book of James reference 5265 below
And they [home town folk] spoke, wondering, (not knowing that He was the son of a virgin, or not believing it even if it was told to them, but supposing that He was the son of Joseph the carpenter,) “is not this the carpenter’s son?”5262 And depreciating the whole of what appeared to be His nearest kindred, they said, “Is not His mother called Mary? And His brethren, James and Joseph and Simon and Judas? And His sisters, are they not all with us?”5263 They thought, then, that He was the son of Joseph and Mary. But some say, basing it on a tradition in the Gospel according to Peter,5264 as it is entitled, or “The Book of James,”5265 that the brethren of Jesus were sons of Joseph by a former wife, whom he married before Mary. Now those who say so wish to preserve the honour of Mary in virginity to the end, so that that body of hers which was appointed to minister to the Word which said, “The Holy Ghost shall come upon thee, and the power of the Most High shall overshadow thee,”5266 might not know intercourse with a man after that the Holy Ghost came into her and the power from on high overshadowed her. And I think it in harmony with reason that Jesus was the first-fruit among men of the purity which consists in chastity, and Mary among women; for it were not pious to ascribe to any other than to her the first-fruit of virginity.