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As a member of the pattern recognition receptor (PRR) class, scavenger receptor class B type 1 (SRB1) plays a key role in innate immunity. Grass carp (<i>Ctenopharyngodon idellus</i>) ranks among the most extensively cultivated freshwater aquaculture species in China. However, little is known about the function of SRB1 in <i>C. idellus</i>. In this research study, a <i>SRB1</i> gene was identified in <i>C. idellus</i>, named <i>CiSRB1</i>. The full-length cDNA of <i>CiSRB1</i> is 2486 bp long, with an open reading frame (ORF) of 2486 bp encoding a 497 amino acid (aa) protein containing a conserved CD36 domain. The identified genomic DNA length of <i>CiSRB1</i> is 20,042 bp, including 12 exons and 11 introns. The predictive analysis of protein interactions revealed that <i>Ci</i>SRB1 could interact with the outer capsid proteins of typical GCRV strains. The tissue distribution of <i>CiSRB1</i> exhibited age-dependent characteristics. <i>CiSRB1</i> displayed the highest expression in the intestines and moderate levels in muscle, spleen, liver, and brain of one-year-old grass carp while maintaining relatively low levels in three-year-old grass carp. Following grass carp reovirus (GCRV) infection, notable upregulation of <i>CiSRB1</i> transcripts was observed in major immune tissues (gills, intestines, spleen, and liver). Furthermore, significant differences were found between one-year-old and three-year-old grass carp, with lower <i>CiSRB1</i> expression levels being detected in the older group. Additionally, a distinct response to GCRV infection was observed in one-year-old and three-year-old grass carp. It was found that one-year-old individuals had a mortality rate of up to 84% 6 days post-infection (dpi), whereas all three-year-old counterparts survived after GCRV infection. The analysis of GCRV copy numbers across tissues revealed substantially higher levels in one-year-old grass carp compared with their older counterparts, confirming the existence of age-dependent susceptibility to GCRV infection in grass carp. Combined with these results, it was speculated that the decline in cell-surface <i>CiSRB1</i> expression with age may impede reovirus binding to host cells, potentially explaining why older grass carp demonstrated enhanced resistance to GCRV infection. This observation accentuates the importance of <i>CiSRB1</i> in the context of GCRV infection and provides insights into age-dependent susceptibility to reovirus.