Complex sentence: He joined his uncle's shipping firm and advanced rapidly by his own merit, becoming a partner by the time he was only 21.while still only a teenager train was instrumental in the building of some of the world's largest and fastest ships for the firm.
Split into simpler sentences: He joined his uncle's shipping firm and advanced rapidly by his own merit, becoming a partner by the time he was only 21. While still only a teenager train was instrumental in the building of some of the world's largest and fastest ships for the firm.