This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
If the items contained in the ArrayList implement Comparable and if the compareTo method uses the age as the basis for comparison, you're set, simply call Collections.sort(yourArrayList) on your array list. If not, then you'll want to create a Comparator class that has a compare method that uses age as a basis for comparison and use an object of that class in the Collections.sort(yourArrayList, yourComparatorObject) method overload that takes a Comparator. Have a look at the Collections class and you'll see what I mean.