Often linked with older age; isn't used much for young people unless they're severely ill. Grandmother's growing infirmity has made stairs difficult for her.
Pro Tip 2/3
Physical Context
Mainly refers to physical strength, not mental capabilities or emotional states. His infirmity since the accident has made it hard to lift objects.
Pro Tip 3/3
Not Disability
Infirmity suggests a condition that can improve, unlike a permanent disability. Her infirmity was temporary, and with therapy, she began to recover.