It is not just common courtesy, it is part of every human's obligation under the fifth commandment.
Leviticus 19:32 Thou shalt rise up before the hoary head, and honour the face of the old man, and fear thy God: I am the LORD.
The phrase "hoary head" is essentially the concept of a head covered by frost - i.e. a head with white or gray hair. The concept of "rise up before" means to stand when they enter the room, which was a sign of respect in those times - and continues to be a sign of respect in many places even to the present day. Likewise, to honor the face of the old man means to be respectful and to show that respect.
The "Western" societies all too often forget about this command. Increasingly, the old are disrespected and even disdained. This is contrary to the law of God. As Christians, we are called to show respect for the elderly and to honor them. This is a general relation between the young and old, which (like the distinction between the sexes) has a heightened emphasis within the family unit. One's elderly father or mother is especially to be honored, just as a woman is to show a particular respect to her own husband. The same goes for children and adults. Children are to be deferential to adults, but they show a particular degree of deference to their own parents.
Finally, let me provide a reminder to any of my readers who happen to be old:
Proverbs 16:31 The hoary head is a crown of glory, if it be found in the way of righteousness.
May I encourage you so to be found!