Not sure how old Rahula was when he became enlightened, but I think he may have been older than 7 years:http://www.palikanon.com/namen/r/raahula.htm
But Seven years seems to be the youngest age for a few of them:
When I was seven & newly gone forth, having conquered with my power the great powerful serpent, I was fetching water for my preceptor from the great lake, Anotatta, when the Teacher saw me & said: "Look, Sariputta, at that one, the young boy coming there, carrying a pot of water, well-centered within, his practices — inspiring; his bearing — admirable. He's Anuruddha's novice, mature in his powers, made thoroughbred by a thoroughbred, good by one who is good, tamed by Anuruddha, trained by one whose task is done. He, having reached the highest peace & realized the unshakable, Sumana the novice wants this: 'Don't let anyone know me.'" http://www.accesstoinsight.org/tipitaka ... .than.html
The other monks all did the same, until at last only a seven-year old novice, who was also an Arahant, remained. The novice said that he was inexperienced, but Potthila insisted so the novice gave him this instruction:
“Venerable sir. There are six openings in a mound, which an iguana makes his home. If you want to catch the iguana, close up the five exits from the mound, and wait for it to come out from the last exit. There are six doors through which sense-objects can enter. If you close five of them, and keep watch at the mind-door, your task will be accomplished.
What the young Arahant advised was for Potthila not to allow impulsion to hang on to any of the five sense-doors, but to shut them all, and note only the mind-door so that impulsion could lead him on to insight. This gave the learned monk a clue to the method of insight practice. When one sees, one must stop at the thought moment of determining and note all phenomena with mindfulness. It is the same as saying, “When you see, you just see it.” Having practised meditation as suggested, Potthila attained Arahantship.http://webcache.googleusercontent.com/s ... clnk&gl=au