Because most of them start training at very young ages. Most of them started training for battle at age 7.
Why Were The Romans So Successful In Battle?
Romans were highly skill-full thanks to their training. It was well disciplined and tiring. Soldiers would carry 20kilos of equipment and beginners would carry a sword and shield twice as heavy as the real thing. So all that training became useful in battle, which made them quite successful if you ask me. They were also very organized and had great leadership.