Which of the following conditions does NOT indicate a decreasing rate of nail growth?

Nail growth can be influenced by various factors, and summer is typically associated with an increased rate of nail growth compared to the other conditions listed. During summer months, warm weather and increased blood circulation can enhance metabolic activities, potentially leading to faster nail growth.

In contrast, winter often correlates with a decrease in nail growth due to colder temperatures which can reduce circulation and metabolic rates, as well as the effect of drier air drying out the skin and nails. Age can also lead to slower nail growth, as the body's regenerative processes tend to slow down with advancing years. Health conditions can severely impact nail growth as well, with certain illnesses or deficiencies leading to compromised nail health and reduced growth rates.

Thus, summer stands out as the one condition that does not indicate a decrease in the rate of nail growth, making it the correct answer.