Complex sentence: When he was 20, he joined the military service at Querétaro, where he was in barracks for about a year and also served in the Veracruz and Monterrey regiments, and eventually reached First Sergeant grade.
Split into simpler sentences: When he was 20, he joined the military service at Querétaro, where he was in barracks for about a year. He also served in the Veracruz and Monterrey regiments, and eventually reached First Sergeant grade.