Simple Summary: The age of horses can influence several properties of the obtained raw material. As the age of horses increases, the meat retains less water, and more fat (p < 0.05) and minerals. In general, horse meat from older animals exhibits undesirable stringiness and hardness, due to a large proportion of connective tissue (collagen). Currently, many methods are applied to improve the tenderness of meat. Of these, the most popular is marinating the meat with various substances, which enhances the functional and sensory properties of the meat. Freezing is a widely accepted method for extending the shelf life of meat. Both the technique used for freezing and further storage at negative temperatures have an impact on some of the properties of meat. Most importantly, the pH value, color, and water absorption of meat tend to change with freezing. In addition, the dry matter content and tenderness of meat increase. This study aimed to analyze the impact of horse age, marinating substances, and frozen storage on the quality of horse meat. As horses age, the values of meat cutting force increase (p < 0.05). For example, the cutting force increases by 4.57 N/cm2 during the first period of freezer storage, and by 3.28 N/cm2 after 3 months of freezer storage (p < 0.05). The present study analyzed the influence of horse age, substances used for marinating, and frozen storage on the quality of horse meat. It was conducted on the samples of the longest thoracic muscle, obtained from 12 carcasses of horses (aged 4–7 and 8–12 years). Among the analyzed samples, a higher fat content (p < 0.05) was found in the meat obtained from the carcasses of older horses. The pH value of the meat samples was influenced by the treatment applied (p < 0.05). Of the substances used for marinating, malic acid caused a decrease in the pH of the meat obtained from young horses (p < 0.05). A similar effect was observed with the addition of phosphates to malic acid-marinated meat. On the other hand, the use of phosphates for marinating resulted in an increase in the pH of the meat obtained from older horses (p < 0.05). The substances used for marinating the horse meat did not significantly affect the reduction in cutting force values. Furthermore, the values of shear force, hardness, stiffness, gumminess, and chewiness of the meat increased with horse age (p < 0.05). An influence on the color parameters a* and b* of the meat was found for the interaction between age, storage period, and the type of treatment (p < 0.05). The use of lactic acid and malic acid for marinating the meat of young horses caused a decrease in the proportion of red color (4.67 and 3.43) and an increase in the proportion of yellow color (3.81 and 1.71), especially after 3 months of freezer storage. All the substances used for marinating (except for phosphates) were associated with higher (p < 0.05) thermal and forced drips of meat from the carcasses of both young and older horses during each storage period, in comparison to the control. The interaction between age and the type of treatment had an influence on the tenderness and juiciness of the horse meat (p < 0.05). In sensory evaluation, it was noted that the interaction between age and the treatment procedure influenced the tenderness and juiciness of the meat samples (p < 0.05). There is still a need for further research to increase knowledge regarding how to improve the quality of horse meat, and ultimately increase the demand from consumers and meat processing plants. [ABSTRACT FROM AUTHOR]