The midlife crisis cannot be reduced to a psychological disorder, a cultural narrative, or a contingent life event. Within the DD four-round structure of the Self-as-an-End (SAE) framework, it is the phase-transition window in which Step 2 of Round 4 (14DD, purpose) must leap to Step 3 (15DD, non dubito). This paper derives the structural inevitability of this wall from first principles: Step 2's arrow points back to the self; without an external reference point, the direction can only be self-calibrated, narrowing with each iteration until it hits a dead end. Three types of midlife crisis are distinguished (weak pseudo-14DD / strong pseudo-14DD / authentic 14DD isolation), three philosophical false solutions are analyzed (nihilism / isolationism / existentialism), and a 14DD energy-depletion model is constructed to explain the diverse clinical manifestations. Empirical data (the U-shaped happiness curve, 10-year longitudinal partner responsiveness studies, generativity meta-analyses) provide consistency checks on the a priori derivation. The prescription: 15DD is the sole structural resolution to 14DD's self-referential paradox; Kant's "persons as ends" is the blueprint; philosophical writing is a second-best solution that bridges toward future subjects when no present 15DD channel is available. Chinese is the authoritative language; the English version is an independent rewrite. Keywords Self-as-an-End, SAE, 14DD, 15DD, midlife crisis, phase-transition window, nihilism, existentialism, isolationism, energy-depletion model, Kant, Nietzsche, Schopenhauer, Sartre, Heidegger, generativity, purpose in life Resource Type Publication / Preprint Language Chinese (authoritative), English (independent rewrite) License CC BY 4.0 Related Identifiers (DOI prefix: 10.5281/zenodo) SAE Foundation Paper 1 (.18528813) — IsPartOf SAE Foundation Paper 2 (.18666645) — IsPartOf SAE Foundation Paper 3 (.18727327) — IsPartOf SAE Methodology Paper VI (.19464506) — References SAE Biology Note 1: Metabolic Oncology (.19492773) — IsPartOf SAE Biology Note 3: Eating Disorders (.19501120) — IsPartOf SAE Biology Note 4: Transplant Rejection (.19588656) — IsPartOf SAE Biology Note 5: Depression Phase-Transition Window (.19589573) — IsPartOf SAE Psychoanalysis Series Paper 1 (.19321143) — References SAE Psychoanalysis Series Paper 4 (.19321534) — References Version 1.0 Files to Upload SAE Biology Note 6:中年危机的先验结构.pdf (Chinese authoritative version) SAE Biology Note 6- The A Priori Structure of the Midlife Crisis.pdf (English independent rewrite) Both versions are included in the upload.