Blubber is a critical component of thermoregulation for marine mammals, particularly for cetaceans. However, the cost of overcoming blubber's buoyant force during descent could constrain blubber deposition. One- to 12-year-old healthy, free-ranging common bottlenose dolphins (Tursiops truncatus) were studied in Sarasota Bay, Florida, during summer (mean water temperature: 29.7°C ± 0.1 SE) and winter (mean water temperature: 19.2 ± 0.4°C) to examine ontogenetic and seasonal trends in morphology and blubber deposition. Surface-area-to-volume ratio decreased significantly with age. During summer, yearlings had significantly thicker blubber than 2- to 12-year-old animals but this difference diminished by winter because blubber deposition in response to the colder water temperature was smaller in yearlings (2-mm increase) compared to 2- to 12-year-old animals (3- to 6-mm increase). During summer, buoyancy was highest in yearlings (6.24 N ± 0.41 SE), compared to a buoyant force of −0.98 ± 0.90 N (neutrally buoyant) for 12-year-old animals. Conversely, all dolphins converged upon a similar buoyant force (8.01 ± 0.56 N) in winter. The elevated buoyancy of yearlings in summer presumably limits seasonal blubber adjustments, because all yearlings (regardless of season) converged upon a similar calculated mass-specific cost of descent that was greater than all other age classes. Balancing energetic demands of thermoregulation and locomotion may limit the flexibility of yearlings to adjust blubber deposition in response to fluctuating water temperatures.