There were no effects of Reminder condition before the reminder session (i.e., conditioning and extinction), hence these sessions represent data pooled across Reminder condition. (A) While all mice increased their CS-elicited freezing across conditioning trials, aged mice with access to running wheels freeze more when compared with young mice sexed as males (A) and females (D) (*P<0.05 main effect of Treatment across entire session with post-hoc Tukey’s comparisons). (B) Aging elevates CS-elicited freezing levels during extinction in males (B) and females (E) (*P<0.05 main effect of Treatment across entire session with significant post-hoc differences). In addition, males overall display greater fear retrieval in the beginning of extinction followed by a steeper extinction curve (i.e., accelerated extinction learning) compared to females (^#P<0.05 indicated in the sex that displayed more freezing at that particular block, Sex × Block interaction with significant post-hoc effect of Sex), however, there was no sex difference by the final extinction block. (C,F) Test data of 45 tone presentations with 5 tones per block. Boxed inset represents CS-elicited freezing when averaged across all 45 tones. Aged groups showed elevated overall CS-elicited freezing than young mice (*P<0.05 main effect of Treatment across entire session with significant post-hoc differences). (C) In males, reinstatement is impaired in aged animals living in standard-housing, which is rescued by access to running wheels. (F) In females, reinstatement is impaired in aged mice regardless of access to running wheels (^P<0.05 Treatment × Sex × Reminder interaction with significant post-hoc effect of Reminder). Conditioning and extinction: males (young standard = 13; aged standard = 16; aged running = 12) and females (young standard = 15; aged standard = 14; aged running = 14). Re-extinction/reinstatement test: males (young standard no shock = 7, young standard shock = 6; aged standard no shock = 7, aged standard shock = 9; aged running no shock = 8; aged running shock = 4) and females (young standard no shock = 7, young standard shock = 8; aged standard no shock = 7, aged standard shock = 7; aged running no shock = 7, aged running shock = 7).