The Caledonian species normally breed for the first time from four to six years old. Only once before has a three-year-old eagle been confirmed to lay eggs, and that was in south-east Spain.
It is thought the illegal poisoning of golden eagles may have allowed younger birds to nest in the local areas.
The new information about the breeding was discovered through a satellite tagging project run by the Highland Foundation for Wildlife, Scottish Natural Heritage, Natural Research and the RSPB.
Professor Des Thompson of SNH, who chairs the group running the work, said: "Provided the right conditions now prevail, we hope that these birds will attempt to nest again next year and young will fledge."