Dreads can be locked without being mature, but I think we tend to throw around both words as if they’re interchangeable. One that is locked is completely knotted with no loose undreaded hair showing. Dreadlocks that are mature are probably at least 6 months old, maybe even up to 18 months old before they’re truly mature. It looks smooth and feels rather dense when squeezed. Looking back at old photos, the lox have gotten thicker — a lot thicker than when you first started.
The younger lock appears less solid and is softer to the touch. If you have immature lox I feel pretty sure you’ll know it.