The objective of this work was to investigate whether the C3469T mutation at exon 3 of the pig leptin (Lep) gene is associated with fatness and if the magnitude and direction of these associations is consistent within age. Repeated measurements on body weight (BW), ultrasonic subcutaneous backfat (SB) and loin-muscle (LT) thickness, and intramuscular fat (IMF) content and composition were taken at 160, 180, 215 and 225 days of age on 162 Duroc barrows previously genotyped for the C3469T mutation. Plasma leptin concentration was determined at 180 days. A total of 505 records of IMF content and composition, which were determined both in live samples of longisimus dorsi muscle (LM) and in carcass samples of LM and gluteus medius muscle, were used in the analyses. The genotypic frequencies associated with the C3469T Lep mutation were 0.76, 0.23, and 0.01 for TT, TC, and CC genotypes, respectively. The effect of the Lep genotypes was assessed using a Bayesian approach based on a polynomial mixed model on age. No evidence was found for an association of the Lep genotypes neither with BW, SB thickness nor with plasma leptin concentration. The posterior mean of the difference TT–TC ranged, for LT, from 0.82 (160 days, posterior probability density (PP)> 0 of 87%) to 1.98 mm (220 days, PP > 0 of 96%) and, for IMF, from 0.96 (160 days, PP > 0 of 84%) to − 1.64 mg/g DM (220 days, PP > 0 of 2%). The results suggest an age-dependent association of the C3469T Lep polymorphism with LT and IMF, which may be useful to explain some controversial results in the literature concerning the association of this polymorphism with IMF content.