How steeply metabolic rate scales with body mass has been investigated and debated for a century; however, little is known about scaling within the same individuals as they grow, let alone how environmental variability affects such ontogenetic scaling relationships. We quantified the ontogenetic (within-individual) scaling of metabolic and growth rates of juvenile rainbow trout (Oncorhynchus mykiss) at 10°C after different early-life temperature treatments: (1) 10°C throughout the experiment; (2) 14°C as eggs, then 10°C from hatching onwards; and (3) 10°C as eggs, then 14°C from hatching to completion of the yolk-sac stage, then back to 10°C (10°C being the optimal temperature and 14°C simulating an early-life warm-spell). The steepness (scaling exponent) of both metabolic and growth rate scaling varied significantly but modestly among individuals. No difference was found in the scaling of standard metabolic rate (SMR) between treatments, but maximum metabolic rate (MMR) and aerobic scope (AS) scaled shallower in the 14°C-yolk group, whereas growth rate scaled steeper in the 14°C-egg group. This shows that a warm-spell can have lasting and negative carryover effects on juvenile aerobic performance (MMR and AS) if warming occurs in the yolk-sac stage, and induce compensatory growth if warming occurs in the egg stage. Correlations between metabolic and growth scaling exponents for individual fish further indicated that compensatory growth did not come at the expense of an elevated SMR but traded off with MMR and AS. The specific ontogenetic timing of temperature variability thus affects ontogenetic scaling of metabolic and growth rates differently and independently.