This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan 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.