Experience plus physical peak.
I'd say guys like Max Holloway, Conor McGregor, and Brian Ortega for example, who jumped right into the pros fairly young, as long as they don't eat too many shots and wear their chin out, will have their physical peak coincide fairly well with ample experience. Not all fighters can hit that.
This may be especially challenging for heavier fighters who seem to to knock each other out cold more often, and fighters under LW division, which require more speed and stamina.