The global aging population has heightened the need for nutritional solutions tailored to elderly individuals. In South Korea, seafood consumption among adults aged 65 and above has increased over the past decade, accounting for approximately 12% of total animal protein intake. Despite a growing preference for meat, seafood remains an important protein source for the elderly population. However, many older adults with reduced chewing and swallowing abilities experience difficulty consuming conventional fish products. To address this challenge, this study aimed to develop a senior-friendly alternative seafood that meets both the nutritional requirements and textural accessibility of elderly consumers. To achieve this, 3D-printable fish analogues mimicking the texture and rheological behavior of real fish were developed using marine-derived ingredients as sustainable structuring materials. Composite gels were formulated with White Auxenochlorella protothecoides, methylcellulose, flaxseed oil, and seaweed-derived polysaccharides (agar, iota-carrageenan, and kappa-carrageenan). Textural, rheological, molecular, and microstructural analyses were conducted to evaluate the physicochemical characteristics of the developed gels compared to those of real fish. Among the three gelling agents, agar-based gels exhibited the most similar viscoelastic response and structural uniformity, confirming their superior suitability as marine-based structuring materials. Further analysis revealed that agar concentration played a key role in modulating the mechanical properties of the gels, particularly hardness. This tunability demonstrates the potential of agar-based gels for designing senior-friendly foods, in which hardness is a key determinant defined by the KS H 4897 standard. Based on these findings, printable inks were formulated to achieve hardness levels corresponding to the tongue-, gum-, and teeth-intake categories. Moreover, all samples exhibited pronounced shear-thinning behavior and high structural recovery, confirming their suitability for extrusion-based 3D food printing systems. This study presents a framework for developing senior-friendly alternative seafood through the control of marine-derived hydrocolloids, demonstrating agar’s tunability as a structuring material and offering practical insights for 3D food printing aimed at personalized food applications in the elderly.