Hajj is obligatory once in a lifetime only upon a Muslim who is sane, has reached puberty, and possesses istita'ah — the means to perform it. The Quran ties the obligation directly to capability: "...and [due] to Allah from the people is a pilgrimage to the House — for whoever is able to find thereto a way" (3:97).
Scholars unpack istita'ah into several elements: physical fitness to undertake the journey (or, if genuinely unable, arranging for someone else to go on one's behalf); a safe and viable route; and financial sufficiency — enough to cover round-trip travel, accommodation, and living costs during the journey, over and above one's basic needs, debts, and the ongoing support of one's dependents while away. A person is not considered obligated if fulfilling Hajj would mean neglecting family responsibilities or taking on unsustainable debt. Scholars differ on timing: Hanafi and Hanbali jurists hold the obligation is immediate once istita'ah is met, so delaying without a valid excuse is sinful; Shafi'i jurists consider it obligatory on a lifetime timescale, permitting reasonable delay so long as one intends to fulfil it.