In the Bible, the Word of God, when talking about the ages of people, “young” is defined as “under the age of 60.” “Old” is defined as “age 60 or over.”
When you read through the Word of God, you will notice that every person who has not yet reached their 60th birthday is referred to as “young,” and every person who is 60 years of age or older is referred to as “old.”
This makes perfect sense because Genesis 6:3 says,
“And the LORD said, ‘My Spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.’”
Since God has promised man a 120-year life (see How Old Can a Christian Live to Be?), 60 years of age, therefore, would be the halfway point in a person's life. The first half of a person's life they would be “young,” and the last half of their life they would be “old.”
It should be pointed out here, however, that there is nothing negative about being “old!” It only means that you are not “young.” It doesn't imply that you are weak. It doesn't imply that you are sick. It doesn't imply that you are losing your hair or your teeth or your hearing or your vision or your mind or anything. “Old” just means that you are 60 years of age or above.
I Timothy 5 talks about being young and old. Verse 1 says,
“Rebuke not an elder, but intreat him as a father; and the younger men as brethren;”
We know that in the New Testament the word “elder” is used two ways. If the context is referring to the ministry, an elder is a pastor. But otherwise, an elder is a person who is older. It is clear from the context here that the ministry is NOT being referred to. An elder here is a person who is older. We see here a contrast. The verse is talking about older men and younger men.
We see the same thing in the next verse that is referring to women. Verse 2 says,
“The elder women as mothers; the younger as sisters, with all purity.”
There is a contrast here also, between older women and younger women.
The next 14 verses deal with how widows are to be treated in the church, but within these verses there is a revelation of who is “young” and who is “old.” Verse 9 says,
“Let not a widow be taken into the number under threescore years old....”
A “score” is 20. So “threescore years old” would be “60 years old.” Then verse 11 says,
“But the younger widows refuse: for when they have begun to wax wanton against Christ, they will marry;”
And verse 14 says,
“I will therefore that the younger women marry....”
Paul says that the “younger widows” or “younger women”—those under the age of 60—are not to be “taken into the number” and supported by the church, because they will probably want to get married. But the “older widows” or “older women”—those age 60 or over—can be “taken into the number” and supported by the church as long as they meet the conditions that are listed in I Timothy 5:3–10.
We see here that “younger” means “under the age of 60” and “older” means “age 60 or above.”
Sometimes the Bible uses the word “aged” (AY-jidd) in referring to “old” people. “Aged” means “age 60 or over” just like “old” does. II Samuel 19:32 says,
“Now Barzillai was a very aged man, even fourscore (80) years old:”
We also see the same contrast between “young” and “aged” as we see between “young” and “old.” Job 29:8 says,
“The young men saw me, and hid themselves: and the aged arose, and stood up.”
Titus 2:2–3 talks about the “aged men” and “aged women.”
2 That the aged men be sober, grave, temperate, sound in faith, in charity, in patience.
3 The aged women likewise, that they be in behaviour as becometh holiness, not false accusers, not given to much wine, teachers of good things;
It then contrasts them to the “young women” and “young men” in verses 4–6.
4 That they may teach the young women to be sober, to love their husbands, to love their children,
5 To be discreet, chaste, keepers at home, good, obedient to their own husbands, that the word of God be not blasphemed.
6 Young men likewise exhort to be sober minded.
Let's look at I Kings, chapter 12. Rehoboam (ree-hah-BOH-amm) was the 4th king of the nation of Israel. (He was also the 1st king of the southern kingdom of Judah, because after being king of the nation of Israel for only a few weeks, the kingdom split in two and the northern kingdom became known as “Israel” and the southern kingdom became known as “Judah.”) In verses 3–5, before the nation of Israel split in two, the Bible says,
“Jeroboam (jer-ah-BOH-amm) and all the congregation of Israel came, and spake unto Rehoboam, saying, ‘Thy father (King Solomon, the 3rd king of the nation of Israel) made our yoke grievous: now therefore make thou the grievous service of thy father, and his heavy yoke which he put upon us, lighter, and we will serve thee.’ And he (Rehoboam) said unto them, ‘Depart yet for three days, then come again to me.’ And the people departed.”
Then verse 6 says,
“And king Rehoboam consulted with the old men, that stood before Solomon his father while he yet lived....”
Notice that the men that stood before Rehoboam's father, Solomon, are referred to as “old men.” That means that they were at least 60 years of age. This fits because Solomon reigned for 40 years as king, and when he died and Rehoboam became king, the men that had stood before Solomon would have been more than 60 years old by this time.
Then verse 8 says in referring to King Rehoboam,
“But he forsook the counsel of the old men, which they had given him, and consulted with the young men that were grown up with him, and which stood before him:”
The Bible calls the men that grew up with Rehoboam “young men.” Rehoboam was 41 years old at this time (I Kings 14:21). This means that “the young men” that grew up with Rehoboam must have been about the same age as he was (in about their early 40's) or they could not have grown up with him! Therefore a person in their 40's is a “young man” or “young woman.” A person in their 50's is also a “young man” or “young woman.”
The Bible does not use the term “middle-aged.” The Bible only refers to people as “young” or “old” (or “aged”). However, if we, Christians, were to use the term “middle-aged,” we would say that a middle-aged person would be a person between the ages of 50 and 70.
In the non-Christian world, a middle-aged person is about 35–40 years of age. The Bible tells us in Psalm 90 that people that live in sin live on average about 70 or 80 years. Some live a little longer, but some don't even make 70, so the average is about 70 or 80 years.
Note: There is another revelation within the 14 verses (I Timothy 5:3–16) that talk about how widows are to be treated in the church. Not only does Paul suggest that the younger women (those under the age of 60) marry, but he also talks about them bearing children. This would suggest that those over the age of 60 would be past child-bearing age. This also makes perfect sense. We know that women normally reach the end of their child-bearing years before the age of 60. God made women this way. This is because God does not want a woman to give birth to a child and then die a few years later so that the child has to grow up without their mother. If a woman—who was living out her full 120-year life—could get pregnant and have children right up until the day of her death, many children would be forced to grow up without their mothers.
The Word of God promises men and women long life. The Word of God talks about living to see your grandchildren and your great-grandchildren. Since God has promised us a 120-year life, and we know that a woman's child-bearing years are over before she reaches 60 years of age, that would leave more than 60 years for a woman's oldest child to grow up, get married, have children, and have their children grow up and get married and have children.