Вот исправленная версия вашего скрипта. Было две проблемы
@echo off
set /p age="enter your age:"
if %age% leq 50 (
if %age% geq 20 (
echo "You are %age% years old in the 21st century."
) else (
echo "You are not in range."
)
) else (
echo "You are not in range."
)
pause