Introduce the younger to the older, then the older to the younger (respect for age). If there was a gender difference, you would introduce the man to the woman first. For example:
John is 23.
Jack is 19.
Katie is 22.
Order:
Katie, this is John, and this is Jack.
John, this is Katie, and this is Jack.
Jack, this is Katie, and this is John.
Think of something you can say about each guy. Like maybe Jon owns a bar and Charles is your mechanic. You have to know something about each person right?
You can then introduce them "Hey this is my friend Jon, he owns the local Chili's down the street"
"And, this is Charles, he was my is my mechanic. If you ever need something fixed, he is the man".
No need to get formal or put together math equations based on trivial things such as their age.. they're your friends, not royalty. Just say something like "Hey Jon, this is Charles. Charles, that's Jon." There aren't too many other ways to go about it.