All sunfish have a lateral line. It's the nerves along their upper sides below their back that enable them to sense vibration. You're referring to the lateral stripe, PH. I've seen juvenile bass fry with pronounced lateral stripes, so I don't think it's an age deal. More of a water clarity deal plus how deep the fish was and what type of cover it is utilizing. Grass fish from clear water will have a much more pronounced lateral stripe than a fish from muddy water.
"Things turn out best for those who make the best of the way things turn out" - Zachary Troy Schrah - a young man with vision far beyond his years.