If you do a Bible search for the phrase age differences in relationships, you will find exactly 0 results. In fact, a person’s age is rarely mentioned in Scripture, and this holds true for married couples in the Bible. We just don’t know of couples’ age differences in the Bible.
Abraham and Sarah are the exception; we do know the age difference between those two. When God promised Abraham that he and Sarah would be the start of many nations, “Abraham fell facedown; he laughed and said to himself, ‘Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?’” (Genesis 17:17). Based on Abraham’s mirthful questions, he and Sarah were ten years apart—not a huge age difference, but noteworthy. There are no other couples in the Bible where both individuals’ ages are given.
It is often assumed that Boaz was significantly older than Ruth. This is based on a passage in Ruth 3. When Ruth asks Boaz to be her kinsman-redeemer, Boaz replies, “The Lord bless you, my daughter. . . . You have not run after the younger men, whether rich or poor” (verse 10). The implication is that Ruth, rather than seeking a husband among those closer to her own age, either in Moab or in Israel, sought to follow Jewish custom and place herself under the protection of the more mature Boaz. The Jewish Mishnah sets Boaz’s age at 80 and Ruth’s at 40 (Ruth Rabbah 7:4; Ruth Zuta 4:13), but that’s pure speculation, since the Bible does not reveal the age difference between the two.
It’s also commonly thought that Joseph was significantly older than Mary. However, there is absolutely nothing in the Bible to indicate that.
Given the Bible’s silence on the issue, we can say that, in the end, age differences in relationships are not a major concern to God. Age can be important in a marriage, of course, but it is far less important than other issues such as salvation, spiritual maturity, compatibility, etc. As people get older, age differences mean less and less. Obviously, a 40-year-old marrying an 18-year-old will raise some eyebrows, but no one thinks twice about an 82-year-old marrying a 60-year-old.
The person we marry should be of the opposite gender (Genesis 2:21–25), and he or she should be a believer in Christ (2 Corinthians 6:14). And, of course, we should marry for the proper reasons (lust and greed being improper reasons). Beyond those guidelines, we have a certain amount of freedom concerning whom to marry. Age differences are a matter of consideration, and we should certainly ask for wisdom when the difference is great (James 1:5), but the Bible does not treat age differences as a moral or spiritual issue.