According to Boston Marathon 2025 statistics, about 9% of entrants don't start the race.

Number of male and female entrants in the 18-39 age group are almost even, but the proportion of female entrants drops in the older age groups.

This pattern appears in 2024 and 2023 as well.

see https://registration.baa.org/2025/cf/Public/iframe_Statistic...